SMITH: DirosCOREA VILLOSA 551 
A careful study of serial sections of embryos of Dioscorea 
villosa, corresponding in size to those figured by Solms-Laubach, 
shows that no ring-primordium is formed, but -that the first 
secondary leaf and cotyledon originate as limited areas of meri- 
stematic activity in an embryo which is still meristematic in all 
its parts; there is a very great increase in the size of the 
embryo as a whole after the first secondary leaf and the coty- 
ledon have begun their development (Fics. 15-24, all drawn 
on the same scale). The growing point of the stem consists 
merely of a group of cells in the axil of the first secondary leaf, 
and remains quite undifferentiated (Fic. 12, g) until germination 
occurs. The growth of the first secondary leaf is more vigorous 
at the start than that of the cotyledon (Fic. 15); but the latter 
soon begins to elongate rapidly, then expands to form a foliaceous 
structure which, in the fully-developed embryo, is peripherally 
very thin and flat, but is much thicker in the middle portion and 
at the base (Fics. 13, 24). The first secondary leaf continues 
to swell out, then arches over and finally covers the growing point 
of the stem (Fic. 24). In the full-grown embryo, the edges of the 
base of the cotyledon extend for a short distance over the first 
secondary leaf, but by no means cover it (Fic. 13). A strikingly 
different structure has been described for embryos of other mem- 
bers of the Dioscoreaceaé, which have a distinct sheath entirely 
covering the first secondary leaf. Beccari (1870a) finds this sheath 
entire in the genera Dioscorea and Trichopus, and two-parted in 
Rajania and Tamus; Solms-Laubach (1878) says that it is entire 
in Testudinaria. The short, thick suspensor persists to the time 
of the maturity of the seed (Fic. 13, s). 
A vascular system, consisting of procambium strands, is present 
in the full-grown embryo of Dioscorea villosa. Cross-sections 
through the hypocotyl show a solid plerome cylinder which be- 
comes a hollow cylinder at the base of the cotyledon. The hollow 
cylinder opens out into a trough-shaped mass as it passes into the 
cotyledon, then branches into three main bundles, which, in turn, 
branch profusely (Fic. 13). The first secondary leaf has a well- 
marked median bundle and two rather weakly developed lateral 
bundles. Bucherer (1889) describes but one vascular strand in 
the cotyledon of Tamus communis; in Tamus, however, the 
