June, 1923 ] 
BLODGETT — EMBRYO OF LEMNA 
339 
the time the seed is fully mature, the root has become about twice as long as 
the diameter of its base, and forms a conical elongation quickly recognized 
as a root structure. 
While the root initials of the daughter frond have been developing into 
the first root structure, the secondary frond-rudiments have been laid 
down at the base of the daughter frond, and are being enclosed by sheathing 
overgrowths from the adjacent parent tissue. This means that there are 
discernible in a seed prior to germination, in this species of Lemna, three 
generations of leaf structure: L 1 , the plumule; L 2 , its daughter frond, 
enclosed in the basal pouch; and L 3 , the paired rudiments at the base of 
the daughter frond, each within its pouch (a and b , text fig. 1). By the 
time germination is under way, these also will have their daughter out¬ 
growths laid down, and thus an additional generation of outgrowths will 
be present before the separation of seed coats from seedling is accomplished. 
But no definite apical region appears at any time; only the successive out¬ 
growths show the presence of cells functioning as apical meristem in the 
region of insertion of the new fronds upon the older tissues, from which 
additional new outgrowths are put forth as if axillary to the last preceding 
structures in each pouch. In mature plants these additional outgrowths 
may be developed behind (“axillary” to) flowers, as well as behind vegeta¬ 
tive fronds. 
Germination 
The ripe seed—or fruit, as the carpel usually persists about the seed 
itself—may be forced from the frond pocket by impact of rain or other 
surface disturbance, by the development of an outgrowth from the tissue 
at the base of the pouch as just mentioned, or, after the death of the frond, 
it may be released by decay. Under normal conditions the seeds may be 
found floating before germination is visibly begun, and at this time they 
lie at the surface upon their sides. Upon the actual inception of germina¬ 
tion, the micropylar end of the seed becomes lighter, by virtue of the spongy 
character of the rapidly enlarging sheath tissue of the embryo, and this end 
comes uppermost, the seed now being vertical in the water. The operculum, 
previously mentioned as formed by the tips of the two integuments of the 
seed, is forced off, leaving the end of the seed open for almost its full diam¬ 
eter. Through this opening the sheath and enclosed plumule emerge, and 
the plumule promptly assumes its horizontal position on the water surface. 
At this stage the seed, protruding sheath, and emerging plumule together 
have the appearance of a diminutive railway spike, the seed forming the 
shaft, the sheath and partially protruded plumule the one-sided head, 
thick at the attachment and diminishing to a tongue-like margin. 
The elongation of the tissues involved is chiefly by an expansion of 
existing cells rather than by a development of new ones, and this elongation 
is localized mainly in the region immediately adjacent to the insertion of 
the plumule upon the cotyledon or hypocotyl (fig. 7). The haustorial 
