June, 1923 ] 
BLODGETT — EMBRYO OF LEMNA 
337 
Plate XXVIII, the inner cells of the operculum are approximately cubical in 
form, while those of the outer layer become elongated to resemble palisade 
cells in proportions, but are not of equal size in all parts of the cap. 
Inception of the Stem Apex 
At the stage when the position of the stem apex can first be located, the 
embryo is about twice as long as wide and broadly pyriform in shape. The 
apex is recognized by the appearance of a low mound of cells about midway 
of one side from base to tip of the embryo, which rapidly increases in size 
until nearly or quite hemispherical. The cells of this region differ in no 
evident way from those of the rest of the embryo. At this stage the embryo 
is 4 to 6 cells in diameter and from 12 to 15 cells in length. Figure 2 shows 
a stage slightly older, with the cotylar sheath making its appearance as a 
collar about the base of the apical mound. The anterior region of the 
embryo from this stage rapidly increases in diameter and carries with it 
the insertion of the apical mound, thus turning the axis of the mound 
through nearly 90 degrees, from a lateral protrusion to a position closely 
appressed to the hypocotyl. Meanwhile there is formed about the base of 
the apical mound a collar, which eventually completely encloses it, except 
for a pore left at the tip of the sheath so formed ( 5 , fig. 4). From this stage 
it will be convenient to speak of the apical region as the “ plumule,” as it 
develops directly into the primary leaf structure, and of the enclosing wall 
as the “cotylar sheath,” since it is formed from the tissues at the base of 
the cotyledon. The hypocotyl (FT, fig. 4) is fused of course to the sheath 
tissues, and is indistinguishable from the rest except by position, and in 
sections such as shown. There is at no time a development of any structure 
resembling a radicle by the embryo in this region, and in the species studied 
there is none developed by the plumule, although in L. minor , as shown by 
Hegelmaier’s 3 figures, a root is developed by the first leaf (plumule); this 
statement is copied in the recent paper by Goebel 4 and evidently confirmed 
by others for the same species. 
The relation of parts in a well advanced embryo of Lemna is much the 
same as in an anatropous ovule, with the plumule taking the position of 
the nucellus, the cotylar sheath that of the integuments, the base of the 
cotyledon corresponding to the chalazal region, and the hypocotyl to the 
raphe of the ovule. In the fully grown seed, the plumule and cotylar sheath 
are nearly one half the full length of the embryo, the anterior half, or 
slightly more, of the embryo being true cotyledonary tissue and functioning 
at germination as an haustorial organ quite as definitely as does the scutellum 
in Gramineae, which latter has been compared to the “foot” of Selaginella 
3 Hegelmaier, F. Die Lemnaceen. Leipzig, 1868. 
4 Goebel, K. Zur Organographie der Lemnaceen. Flora 14: 278-315. 1921. A 
discussion of views by Lotsy, Hegelmaier, and S. Rostowzew (Russian) on L. minor , with 
copied figures from three sources. 
