Gardeners' Chronicle - May 3, 1890
The Microscopic Structure of Hybrids
Dr. J. Macfarlane
Botanic Laboratory, Royal Botanic Garden, Edinburgh

At the March meeting of the Edinburgh Botanical Society, I gave a microscopic demonstration of the structure of hybrid plants and of their parents; at the same time I made a few explanatory remarks, and I propose now to expand these into a short communication. I trust that this may prove interesting to those at least of your readers who look to hybridisation as to the great field of promise for experimental horticulturists.

During the last few years I have studied minutely the general and microscopic structure of pitchered insectivorous plants. At an early stage in my investigations, I was struck by the perfect blendings in certain well known hybrids of the appearances presented by their parents, and this not merely in habit, consistence, shape and colour, but even in such minute details as the relative number of stomata in a given area, the size and shape of cell hairs and of the cells from which these sprung, and the mode of deposition of thickening substance on their primary cell wall.

The fine series of hybrid Nepenthes, raised by the late Professor Dickson and Mr. Lindsay, or presented to the Royal Botanic Garden by the Messrs. Veitch, formed a rich field for study, but my attention was specially arrested on examination of a set of seventeen hybrid Sarracenias, kindly supplied to me some months since by Mr. Moore, of Glasnevin Garden. As one after another of these was passed under the microscope, I was gradually inclined to believe that a hybrid plant may exhibit blending of parent peculiarities in every cell. This was easily demonstrated in the case of epidermal tissues which are apparently the most plastic of all.

Unwilling to trust the evidence of such highly specialised plants, I examined other hybrids belonging to various orders, as Dianthus Lindsayii, Philageria Veitchii, Saxifragia Andrewsii and Churchillii, Hedychium Sadlerianum, and also the supposed doubtful hybrid Bryanthus erectus; these not only verified my previous conclusions, but enabled me to extend them in a convincing way. I now subjoin descriptive notes of the more striking parts which may be readily examined by any one conversant with microscopic work.

The first-named hybrid, Dianthus Lindsayii, is not only intermediate between Sweet William (D. barbatus) and the pretty alpine Pink (D. alpinus) in habit and leaf-form, but if we compare the leaf-epidermis of the three, we find that in the field of a low-power Zeiss objective (A), Sweet William shows about 900 stomata on its under epidermis, and 100 to 400 on its upper; the alpine Pink shows on its under epidermis 600 stomata, and on its upper 460; the hybrid shows 750 on its lower, and 290 on the upper epidermis. The epidermal cells also of the hybrid are intermediate between those of the parents.

Philageria Veitchii is a remarkable cross. With a decided leaning in naked-eye and microscopic appearance to the seed-parent, Lapageria rosea, it nevertheless shows in cells of the root, stem, leaf, and flower a strong infusion of Philesia buxifolia "blood." Thus the thickened cells in the bundle-sheath of fully matured roots of the hybrid are intermediate between the greatly thickened cells of L. rosea and the slightly thickened cells of P. buxifolia. In the stem, the epidermis, cortex tissue, bundles, and matrix-cells exhibit a similar blending; but as usual it is the leaf which presents the most pronounced union of characteristics. On surface view, the upper epidermis of L. rosea shows thin-walled, wavy cells, that of Philesia buxifolia thick and straight-walled cells; that of the hybrid shows thicker-walled cells than the former, and approaching the latter on being nearly straight; the lower epidermis in L. rosea consists of thin-walled, wavy cells, and amongst these are irregularly scattered stomata, whose guard-cells are fully exposed on the surface. In P. buxifolia, the epidermal cells are straight-walled, but have undulating free surfaces, and embedded deeply beneath these many stomata appear, arranged in nearly in parallel rows. The hybrid, though showing a bias towards the female parent, is again intermediate. In the amount of pallisade and loose parenchyma, and the size of the chloroplasts, the same is true.

I have not as yet had opportunities of examining the flowers of many hybrids, but for specimens of that just discussed, I am indebted to my friend, Mr. Dunn, of Dalkeith Palace Gardens. While the echinated pollen grains of the parents appear plump and well filled with protoplasm, those of the hybrid are shrunk and starved looking, with the exception of one here and there, which seem as if they could effect fertilisation. The same apparently holds true so far as I have examined (this includes nine hybrids and their parents), but the more nearly two parent species approach each other in habit and structure the more abundant do the potential pollen grains appear to be.

Saxifragia Andrewsii is a hybrid which perfectly blends the peculiarities of the "Geum" and "Aizoon" sections of the genus. The upper epidermis of the leaf is devoid of ordinary stomata, as in S. Geum and its allies, and in this differs from S. Aizoon and the crusted Saxifrages, which all have an abundance distributed over patch-like areas. The marginal water stomata in size and colouring resemble the former, but the epidermal cells which surround them have little knob-like excrescences on which gathers the calcareous deposit which gives to the crusted Saxifrages their typical character. These are only found in the "Aizoon" group. But the mesophyll tissue and vascular supply in the leaf of the hybrid is also intermediate, for, to refer only to the last, S. Geum has the lowest water stoma on each side of the leaf margin partly supplied by two bundles which spring from the main leaf bundle near its junction with stem, partly by branches of two given off much further up; S. Aizoon has three to five on each side, wholy supplied from the lateral pair; in the hybrid two to three are thus supplied.

The hybrid Hedychiums, in which the late Professor Dickson interested himself greatly, and which were duly described in this periodical, were next examined. I hoped that these would show in the starch granules of the rhizome cells blending of the parent conditions. I was greatly pleased to find my hopes amply realized. Selecting pieces of mature rhizome of H. coronarium, H. Gardnerianum, and the hybrid, I found that while the granules of the first were large, flat, oval plates, and those of the second small triangular shells, those of the hybrid were shaped as if half of a H. coronarium granule had been fused with a reduced one of H. Gardnerianum. A study of the hybrid resulting from H. coronarium crossed by H. elatum gave like results.

While treating of cell contents, I may note that by the kindness of Mr. Buchanan, and of his gardener, Mr. Grossart, I have been able to examine the hybrid Masdevallia Chelsoni [amabilis x veitchiana] and one of its parents. The purplish-red sepals of the former owe their colour to large rich yellow granules or chloroplasts present in the epidermal cells, and to vesicular hair cells springing from the epidermis, which are filled with a purple pigment. In M. Chelsoni the yellow chloroplasts are mostly one-third to one-fourth the size of those found in the parent already examined. I fully expect, therefore, to find the chloroplasts in the seed parent as small as, or smaller than, those in M. Chelsoni, when I have an opportunity of looking into it.

For several reasons, I have given considerable attention to the Ericaceae. The beautiful Bryanthus erectus is a bigeneric hybrid of pronounced type. Totally different as are many of the tissues of the parents, these are blended in the hybrid in a singularly interesting manner. The relative size of the pith and of the individual pith cells, the structure of the phloem and of the outer bark, in stems of equal age, the leaf shape on transverse section, and the floral parts in the hybrid, are all intermediate between the parents. Erica Watsoni (a wild cross between E. ciliaris and E. tetralix) is a very evenly balanced hybrid, but I will only refer in detail to its anther structure. In E. ciliaris the anthers have no tails, in E. tetralix tails about one-twentieth of an inch long are produced from each anther lobe, in E. Watsoni the tails are exactly half as long as in the latter. In E. tetralix each cell of the anther wall is swollen out into a short conical mamilla; in E. ciliaris this mamilla is strongly developed, and is often constricted in the middle. Erica Watsoni has mamillae exactly intermediate in size and shape. I have carefully worked over Erica Mackayii, but will defer discussion of it for the present.

The numerous crosses of Rhododendron effected by Mr. Grieve, of Messrs. Dickson's nurseries; by Mr. Fraser, of Messrs. Methven's nurseries; and by Mr. Lindsay, have further verified the line of investigation. The cross between R. ciliatum and R. Edgworthii greatly resembles the former, but scarely at all the latter, in naked-eye appearance; so much is this the case, that I doubted the accuracy of the supposed parentage when I received specimens of the three from Mr. Lindsay. An examination of the leaf alone sufficed to set my doubts at rest. In the number and mode of distribution of the stomata, the structure and cell contents of the scurf-scales, the distribution and shape of the hairs, and the leaf-structure when seen on transverse section, the minute features on both parents were strongly traceable in the hybrid. R. praecox and R. glauco-ciliatum, are equally deserving the attention of any who wish to follow out for themselves this line of inquiry.

A comparison of the stomatic distribution in many hybrids and their parents shows how constant and regular such minute tissue-modifications may be, specially if one observes previously whether the hybrid inclines more to the pollen or the seed parent.

Thus, under the field of view of No. 7, Zeiss objective, Cypripedium insigne shows 11 to 12 stomata, C. villosum shows 21 to 23, the hybrid 16 to 17, C. barbatum has 3 to 4, C. insigne has 11 to 12, C. Ashburtonae (which more nearly resembles the former) has 6 to 7. Dianthus Lindsayi has already been referred to, and I may further cite Philageria Veitchii, various Saxifrages, Sempervivums, Ericas, Rhododendrons, Daphne, &c.

Enough has been adduced to show that, as the result of union of the fertilising contents of the pollen-tube with the egg-cell of a plant, a new character is assumed by the egg-cell, and that as it grows and divides to form the embryo plant, every cell of that plant inherits the peculiarities of both parents at times in a perfectly balanced way, so far as our limited powers of study can carry us, at times with an evident leaning or bias to one parent. It is, therefore, evident that a wide field for observation and experimental research is open alike to the biologist and horticulturist. For the biologist, many problems bearing on heredity, on cell-history, on the relation of species to each other, on the production of hybrids [bud spots], and on the evolutionary origin of organisms, which have hitherto been greatly confined to the region of speculation, may thus be reduced to accurate scientific study. For the horticulturist, who has already opened the way so much, further incentives to the prosecution of hybridisation experiments present themselves.

About sixty hybrids and their parents have now been examined by me, and I hope in time to publish an account of these, and others which may reach me. May I be allowed to solicit from your wide circle of readers, specimens of such hybrids and their parents as can be spared. A bit of root, stem (rhizome, tuber, or other part, if such is developed), a leaf or two, and flower, will amply suffice, or any of these will prove helpful, specially leaf and flower. It is only by comparison by a wide series of plants that such investigations can be fruitful in good results to the horticulturists. I feel sure that many will be willing to aid me in the way indicated.

I may be allowed to express best thanks to the Directors and Curators of the Kew, Glasnevin, Trinity College, and Edinburgh Botanic Gardens, as well as to several private donors, who have generously supplied me with material.

Macfarlane: Flowering Period and Vigor of Hybrids (1891)

Macfarlane: Minute Structure of Plant Hybrids (1892)

Hurst: Partial prepotance in Paphiopedilum (1900)