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AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
by Schlickum, 5 though he recognizes the difference in cell history. The 
similarity between the foot of Selaginella and the cotyledon of Lemna is 
most clearly seen in the germination stages, when the haustorial cotyledon 
remains within the testa as the sheath and the enclosed plumule protrudes 
from the seed and floats upon the surface of the water. 
The Plumule 
The stem apex develops directly into the plumule, or first leaf structure. 
It can not be separately identified either in the embryo or at later stages, 
and is recognized by the location of rudiments of leaf structures as projec¬ 
tions from the meristematic tissue at the insertion of daughter outgrowths 
upon the parent tissues. This point, as relating to older individuals, was 
discussed in a previous paper. 6 The flattened character of the plumule is 
assumed at an early stage, as shown in figure 5, which is nearly intermediate 
in development between figure 2 and figure 3. A cross section at this stage 
would show a nearly circular embryo, with the plumule approximately 
lens-shaped and enclosed by the sheath tissues on all sides, but the walls of 
unequal thickness. No differentiation at this stage into further structures 
is noticed, but a little later the rudiment of the frond to be developed by 
the plumule as a daughter structure is apparent in sagittal sections, as at 
L 2 , figure 4. 
With the appearance of the rudiment of the second leaf the plumule 
becomes unequally lobed at the base, for the daughter frond develops on 
one side only in the plumule, as compared with the paired development of 
later stages. This is soon enclosed by a sheath from the base of the plumule 
in a manner quite like the ensheathing of the plumule by the cotylar pocket, 
and a similar shifting of direction of the axis of the enclosed structure takes 
place. Although Hegelmaier shows figures of germinating Lemna seeds with 
protruding roots as if from the plumule tissue, no indication of such develop¬ 
ment has been detected in the species under discussion, as a plumule charac¬ 
teristic. The first stage at which any root rudiment is discernible is after 
the daughter leaf (of the plumule) is well developed, and the seed nearly 
full-grown. At this stage a cross section would show structures as in 
figure 6. The root initials are just differentiating from parenchymatous 
tissue at the heavy line near the base of the daughter frond, L 2 of the key 
sketch. This occurs when the frond rudiment is about a dozen cells in 
length, but already nearly covered by the overgrowth of the pouch walls 
from the base of the plumule. The root initials are developed from cells 
in the second layer on the ventral side of the frond rudiment; the surface 
layer forms a sac about the elongating root in later stages, but this is finally 
pierced and then becomes the “root sheath” of the mature fronds. By 
5 Schlickum, A. Morphologischer und anatomischer Vergleich der Kotyledonen und 
ersten Laubblatter der Monokotyledonen. Bibliotheca Botanica 35. 1896. 
6 Blodgett, F. H. Morphology of the Lemna frond. Bot. Gaz. 60: 383-390. 1915. 
