Studies in Heredity as Illustrated by the Trichomes of Species and Hybrids of Juglans, Oenothera, Papaver, and Solanum (1909)
WILLIAM AUSTIN CANNON

TRICHOMES OF SOLANUM VILLOSUM x SOLANUM GUINENSE AND OF THE PURE SPECIES

SOLANUM VILLOSUM

There are 2, or possibly 3, kinds of trichomes in Solanum villosum, all of which are multicellular and glandular (fig. 3, a, b, and c). These for convenience are in this paper called the awn-shaped, the giant awn-shaped, and the big-headed trichomes. The trichomes were observed on the stem, on the lobes of the calyx, and on the ventral surface of the leaves. On the stem and on the calyx all 3 types occurred, with no apparent choice of position, but on the leaf the distribution was as follows: only the awn-shaped trichomes occurred between the veins; all were found on the veins.

FIG. 3.—Trichomes of Solanum pure species and hybrids. Solanum villosum: a, awn-shaped secreting trichome from leaf, region of vein; b, giant awn-shaped secreting trichome from region of midrib, ventral leaf-surface; c, big-headed glandular trichomes from young stem. Solanum guinense: d, big-headed glandular trichomes from ventral leaf-surface; e, awn-shaped non-secreting trichome. Solanum villosum x guinense: f, big-headed glandular trichome from ventral surface of leaf; g, big-headed glandular trichome. (For size of trichomes see text; reduced one-half.)

The awn-shaped trichomes are composed of a single row of cells, usually 5, which constitute the stalk, and a single terminal cell, which is somewhat swollen and contains coarsely granular matter. The stalk-cells are provided with a delicate protoplasmic lining and are hyaline. They measure from 176 µ to 352 µ in length, with the greatest number about 193 µ long.

The giant awn-shaped trichomes are different from the awn-shaped ones mainly in the quality of size; they measure from 792 µ to 1.4 mm. No trichomes were observed of the awn-shaped type which measured longer than the longest awn-shaped trichomes, 352 µ, or shorter than the shortest giant awn-shaped trichomes, 792 µ, for which reason the giant awn-shaped trichomes ought probably to be considered a distinct type.

The big-headed trichome is somewhat curved, so that it is frequently closely appressed to the surface. In structure it is composed of a stalk of 2 cells and a head of an undetermined number of cells, probably 4. The stalk-cells are hyaline; the cells composing the head are densely filled with a coarse granular substance. These trichomes measure from 63 µ to 75.6 µ in length.

SOLANUM GUINENSE

Two types of trichomes were observed in Solanum guinense, of which one is glandular and similar to the big-headed trichome of Solanum villosum, and the other is not glandular and is unlike any trichome seen in the other species (fig. 3, e).

The trichomes were found mainly on the veins of the ventral surface of young leaves and on young stems; the older stems and older leaves were usually glabrous.

The big-headed secreting trichomes were either straight or mostly curved and appressed to the surface. They measure from 67.2 µ to 71.4 µ in length. As in structure, size, and general appearance these trichomes are similar to the analogous ones in Solanum villosum, a further description of them is unnecessary.

The non-secreting trichome (the uncommon type) is broadly awn-shaped, is usually curved, and more or less closely appressed to the surface. It is composed of 3 or 4 cells, which rest on a multicellular base. The trichomes measure from 239.4 µ to 378.0 µ in length.

SOLANUM VILLOSUM x SOLANUM GUINENSE

Two types of trichomes were observed in the hybrid Solanum. These were in all respects like those of the species Solanum guinense, and consequently were the big-headed secreting trichome and the awn-shaped nonsecreting trichome (fig. 3, e and f). The trichomes are on the ventral leaf-surface, where they are confined to the veins. The big-headed trichomes measure from 67.2 µ to 80 µ in length, and the awn-shaped trichomes from 252.2 µ to 316.8 µ in length.

The account above given is from a study of first-generation plants. About a dozen second-generation plants were examined also and a condition quite like that observed in the first generation was found. Measurements of the big-headed secreting trichomes of second-generation hybrids showed that they ranged from 63.0 µ to 71.4 µ on the leaves, and 71.4 µ to 75.6 µ on the stem. The non-secreting awn-shaped trichomes measured from 218.8 µ to 316.8 µ in length.

The general relation of the trichomes of the hybrid and the pure lines of Solanum are given graphically in table 5.

TABLE 5—Comparison of the trichomes of Solanum villosum x Solanum guinense with those of the Pure species.

Kind. S. villosum. The hybrid. S. guinense.
Awn-shaped, glandular 176 µ to 352 µ Absent Absent
Giant awn-shaped 792 µ to 1.4 mm Absent Absent
Big-headed 63 µ to 75.6 µ 67.2 µ to 80 µ
*(63 µ to 71.4 µ)
*(71.4 µ to 75.6 µ)
67.2 µ to 71.4 µ
Awn-shaped, not glandular Absent 252.2 µ to 316.8 µ
*(218.8 µ to 316.8 µ)
239.4 µ to 378 µ

*The numbers in parenthesis refer to hybrids of the second generation.

The data at hand are not sufficient either in amount or in kind to permit the tracing of the influence of the pure lines on the hybrid offspring. This much is apparent, however: the trichomes of the non-secreting awn-shaped type which the hybrid inherits from Solanum guinense are of full, not of half size, as MacFarlane found in certain hybrids investigated by him where unilateral inheritance also obtained.*

*MacFarlane, l. c.


CybeRose note: The plant that Burbank called Solanum villosum was probably S. sarachoides, judging from his description. I don't know whether Cannon examined Burbank's plant, or the "true" S. villosum.

Heiser (1987):
What species might Burbank have used? This could hardly be narrowed down by his description of its habitat since in one place he said the plant was from Europe and in another from Chile. There is one species, however, with greenish mature fruits, Solanum sarachoides, a native of South America, which moreover was established in California before 1900 and could have been available locally to Burbank. This species is a dwarf, procumbent annual that matches Burbank's description of "Solanum villosum." Furthermore, Stebbins and Paddock had pointed out that Solanum sarachoides, a diploid, had frequently been confused with Solanum villosum. That no one, other than W. Watson, had caught Burbank's mistake is somewhat surprising. Certainly Bitter, with his knowledge of Solanum, should have been aware of the discrepancy between Burbank's description and the true Solanum villosum.