174 Sargant and Arber .— The Comparative Morphology of 
will be inserted on these two plates respectively. But in such seedlings as 
those drawn in Text-figs. 9 (p. 170) and 12 (p. 171), where the stem-bud is still 
enclosed in the coleoptile, these roots are mere rudiments in which the 
vascular tissue is not differentiated, and the lateral plates are so far 
embryonic too, that xylem cannot be clearly distinguished from phloem. 
Their presence, however, affects the mesocotylar stele even at this age, and 
III. 
IV. 
Text-fig. 16. Avena sativa, L. Structure of axis in seedling about age of that drawn in Text- 
fig. 12, p. 171. Shown in a series of diagrams representing transverse sections from above downwards. 
I. Plumule surrounded by coleoptile. II. Top of first node. III. Base of first node. IV. Mesocotyl. 
they are later of great functional importance. The anatomy of the young 
node is obscure until the structure of these plates is thoroughly understood, 
and it is therefore worth while to describe them in an older seedling where 
they are better differentiated. 
In the oldest seedling examined, the third foliage leaf is just unfolding, 
and the second internode of the plumule is 2 mm. long. It is enclosed 
within the sheath of the first leaf, which is itself surrounded by the base of 
