164 Sargant and Arber .— The Comparative Morphology of 
—irL 
Text-fig. 3. 
Iris fiseudacorus 
L. Seedling, life 
size. 
Triticum , for instance)—botanists would probably have found no difficulty in 
considering the scutellum as the sessile sucker of the cotyledon, and the 
coleoptile as its sheath. Van Tieghem, indeed, did take this view in 1872, 
and justified it by a bold morphological fiction. He treated the whole of 
the axial region separating the insertion of the scutellum 
from that of the coleoptile as an elongation of the first 
node. On that hypothesis it could not be considered as 
belonging either to epicotyl or hypocotyl, and Celakovsky 
in 1897 (p. 145) embodied Van Tieghem’s view in the term 
mesocotyl. We are using this term because it is convenient 
and generally understood, but we do not accept the hypo¬ 
thesis which gave rise to it. 
Our own view of the nature of the mesocotyl is founded 
on the comparative study of the vascular skeleton in Grass 
seedlings and in other Monocotyledons. The facts on which 
it is based will be described in detail later. We think, 
however, that they will be followed with greater ease if we 
outline our hypothesis at once. 
The cotyledonary sheath of Monocotyledons assumes 
many forms. The simplest case is that in which it is 
nothing more than the expanded base of the cotyledon 
wrapped round the plumular bud. This is very common 
in epigeal species, as in Allium . In hypogeal seedlings 
the expanded base is commonly transformed into a closed 
cylinder (Arum maculatum , L., Veratrum nigrum , L., &c. 
Another variant on this type is illustrated by Iris pseu- 
dacorus , L. (Text-fig. 3), and Commelina coelestis, Willd. 
(Text-fig. 35, p. 217), where the stalk of the cotyledon is bent 
sharply downwards just where it begins to expand into 
a sheath. The effect is sometimes to form a sort of hood 
over the young plumule. 
But in other cases when the stalk is bent downwards 
in this way, the hood is formed by an appendage to the 
expanded base ( Tigridia , Text-fig. 8, p. 168, and Elettaria , 
Text-fig. 30, 1 . p. 110). Such a hood often suggests a pair of 
stipules united along one margin or both. Van Tieghem 
( 72 , p. 271) distinguished this structure as the upper or 
stipular sheath from the lower or basal sheath. Occasionally 
the latter is almost wholly suppressed, while the upper sheath 
is well developed {Kniphofia). 
Now suppose a seedling with a pronounced upper sheath to its 
cotyledon, and no basal sheath at all, the stalk of the cotyledon pointing 
downwards and in close contact with the axis (Text-fig. 4). If we imagine 
Text-fig. 4. 
Diagram of ima¬ 
ginary seedling, to 
illustrate how the 
stalk of the coty- 
don might fuse 
with the hypo¬ 
cotyl. 
