The Gardeners' Chronicle, ser. 3. 58: 196-197 (Sept 25, 1915)
DO MENDEL'S LAWS HOLD GOOD FOR CROSSES BETWEEN SPECIES?
W. R. Dykes,
Charterhouse, Godalming

Until we arrive at a satisfactory definition of what constitutes a distinct species, it is somewhat difficult to discuss the question of the validity of Mendel's laws in cases of crosses between species; but in the meanwhile it may be of some interest to record a few instances of the results of crosses between plants so different that probably every one would admit them to be distinct specifically. It is probably true that hitherto the vast majority of experiments on Mendelian lines have been made within the species, in the sense that plants have been used of what would usually be called the same species, though differing in some one character. Comparatively little seems to have been done towards the elucidation of the laws that govern the results of crosses between species, perhaps for the very simple reason that many of these hybrids are entirely sterile. However, the cause of this very sterility is another interesting problem, the investigation of which, if it were possible, might lead to important results.

Over and over again I have been struck, when I have seen a new hybrid for the first time, by the fact that the characters of neither parent could be said to be dominant over those of the other. It may be, of course, that the characters that I have noticed are not Mendelian pairs, but one would have thought that the presence or absence of a beard in an Iris and the presence or absence of a linear perianth-tube would be examples of such pairs of characters. In each case, however, the hybrid gives a compromise between the characters of the two parents.

It is, of course, not easy to obtain crosses between bearded and non-bearded Irises, and I always remember Sir Michael Foster's answer to my question as to whether he had ever succeeded in obtaining such a hybrid. He took me to see a large clump of a plant which he believed had resulted from the fertilisation of a flower of some form of I. germanica with pollen of I. spuria. The clump was flowerless, and Foster went on to say: "The only thing that makes me think that something must have happened is that that clump has never flowered though I have had it for many years. No ordinary germanica would have remained flowerJess for so long."

I therefore tried in another direction, and crossed the only known bearded bulbous Iris, the Portuguese I. Boissieri, with pollen of I. tingitana, which has no trace of beard. The beard of I. Boissieri consists of long, straggling golden hairs 1/8 to 1/4 of an inch long. In the hybrid this is reduced to a number of hairs distinctly visible to the naked eye, but less than one-sixteenth of an inch long.

The Evansia section of the Iris family, to which, belong the "well-known I. tectorum and I. japonica, is distinguished by the fact that its members have a linear, cockscomb-like crest in place of the beard of the ordinary flag Irises. There has recently been in flower here, together with both its parents, a cross between the crested I. tectorum and the bearded Loppio form of I. Cengialtii. In this case also the result is a compromise, for a distinct short brownish beard springs from the top of an equally distinct pale-purplish crest. (See Gard. Chron., June 18, 1910, p. 399.)

All the members of the Xiphion section of Irises, except I. xiphium, agree in possessing a linear perianth tube between the top of the ovary and the short funnel-shaped base of the flower. In I. xiphium I have never seen any trace cf this linear tube, nor have I ever found it absent from any of the other species. During the present season there have been in flower here hybrids between I. xiphium and I. tingitana, and I. xiphium and I. filifolia. In I. tingitana the tube is over an inch in length, and in the hybrid with I. xiphium it is about half an inch long, while in the hybrid with I. filifolia the half-inch tube of that species is reduced to a short quarter of an inch.

Irises of the sibirica group mostly agree in the possession of hollow stems, a peculiarity not found in any other group. There is one outlying member, I. Clarkei, from the Tonglo ridge in the neighbourhood of Darjeeling. which has a solid stem. The stem of I. sibirica itself has very thin walls, which are, however, stouter in the case of some of the other species. In I. chrysographes, for instance, the central hollow is in diameter only equal to half the diameter of the stem. When I. Clarkei is crossed with I. chrysographes the resultant plant has stems in which the central hollow is almost but not quite entirely closed with pith — yet another instance of compromise between what seems to be a pair of characters, which might be expected to act according to the Mendelian laws.

The spathe-valves of Irises are of great value in separating the species, and few can have failed to notice the entirely scarious and papery spathes of I. pallida, which become entirely dry and white before the buds even appear. Another undoubtedly wild species, I. variegata, has entirely herbaceous spathes, which are either wholly green or green flushed with purple. When these two species are crossed — and hybrids between them exist wild in nature — the resultant plants have spathe-valves which are green in the lower half and scarious in the upper part.

Yet another compromise appeared this year in several batches of hybrids between I. reticulata and I. Bakeriana. The leaves of the former have four sides with a horny ridge at each angle. The sides are unequal in breadth, two being broad and two narrow, and the two narrow sides come next to each other, between the two broad sides. I. Bakeriana is unique in having approximately round leaves with eight projecting ridges set at equal intervals. In the hybrids between I. reticulata and I. Bakeriana the leaves have six ridges, the extra two appearing down the centre of the broader sides.

The question of the colour pigments of Irises is "undoubtedly very intricate and difficult, and colour is notoriously of little value towards the determination of specific rank; but even here a few instances have occurred which show that hybrids between species produce compromises between the parents. 

In the so-called sambucina and squalens Irises there is no difficulty in seeing the struggle between the purple of I. pallida and the yellow of I. variegata. The actual tone and shade depend on the various colour-forms of I. pallida and of I. variegata, from which the hybrids result, and it is worthy of note that the same murky colouring is present in a hybrid which I raised between the purple I. trojana and a form of I. variegata.

An even stranger example of the struggle for the mastery between purple and yellow has recently appeared in a hybrid between I. Boissieri and I. juncea. The unopened bud is brown, a colour resulting from the purple veins that lie close together on a dull yellow-brown ground, produced apparently by purple overlying yellow. The styles are brown and the standards a little more purple than the rest of the flowers.

The brick-red or terra-cotta colour of I. fulva has similarly combined with the blue-purple of I. foliosa to produce a rich velvety claret-purple in the hybrid between these two closely-allied species.

I. Forrestii, a comparatively new Chinese relative of I. sibirica, remains apparently true to the yellow colour, and I. chrysographes is equally constantly a magnificent deep violet-purple with a few gold markings at the throat. Hybrids resulting from the crosses made both ways between these species are identical. The purple is lighter in shade and looks as though there is yellow underlying it, while the patch of gold markings at the throat is considerably enlarged.

All these instances seem to show that the law of dominance in the Mendelian sense is by no means universal, and that it does not always occur in crosses between true species.

Another interesting fact is that, with one undoubted exception, I. chrysographes X I. Forrestii, and the possible exceptions of I. pallida X I. variegata and I. fulva X I. foliosa, all the above-mentioned hybrids have proved to be sterile both to their own pollen and to that of both their parents. The two possible exceptions are cases where the parents are somewhat closely allied, while the still more closely related I. chrysographes and I. Forrestii give hybrids that are readily fertile though their progeny is not yet old enough to flower. From all the many squalens and sambucina forms I have only succeeded in obtaining a few seeds and one seedling from a yellow variety. This recently flowered, and gave a pale lavender self-coloured flower with a large, bright-yellow beard. The spathes were scarious in the upper part and green at the bass. The apparent sterility of the other hybrids is extremely provoking, for it would be interesting indeed to discover what the second generation would give from crosses between a bearded and a non-bearded Iris, and between those which possess and those which lack a linear perianth tube.