24 
The plumule-sheath is, of all the organs of the embryo, the most 
striking and difficult to explain. Three very different theories are 
held regarding its interpretation. To the first we attach the names of 
Hofmeister and Sachs, who regard it as an outgrowth of the so-called 
hypocotyledonary internode. 
The adherents of the second view regard it as an independent leaf 
belonging to the plumule and next in leaf arrangement to the scutel- 
lum. According to this the plumule-sheath, which is directly above 
the scutellum on the same side of the axis, would not correspond to the 
distichous arrangement of grass leaves: but if one regards the epiblast 
as a leaf, i. e., a second cotyledon, then the apparent disagreement is 
explained and the plumule-sheath becomes the third leaf alternating 
with the epiblast. Bruns, who is a strong advocate of this view, sees 
no reason why Hanstein should deny the independent nature of the 
plumule-sheath because of its origin. 
. Let us briefly follow the devolopment of Brachypodium according to 
Hanstein’s investigations. The young embryo is at first a spherical 
mass composed of three with sometimes a fourth smaller cell. These 
cells divide several times in all directions and the embryo takes on à 
club-shaped form, becoming longer and narrower at the base. This 
many-celled body is at this time without distinct internal or external 
differentiation. The lower-most cell, which has now become considera- 
bly divided, later goes to form the suspensor, while the two upper cells 
form the embryo proper. Soon the internal differentiation of the 
radicle can be seen with its dermatogen, periblem, and plerome. The 
second stage of development takes plaee in the external part of the 
embryo. In the wall there oceurs a depression which marks the divi- 
Sion of the seutellum from the hypoeotyledonary part. The tissue 
immediately above this depression is the cotyledon, from which a part 
of the plumule-sheath soon emerges, while that on the lower side forms 
the vegetation point with its later-developed leaves. The projection on 
the upper part of the embryo, i. e., on the upper side of the depression, 
shows again on its upper surface another depression similar to the first, 
thus forming a second projection. At the same time a projection in the 
form of a half eollar has formed on the lower part below the vegetation 
Point. These two projections stand opposite one another like lips. 
Finally they grow and their margins unite to form a complete cap over 
the plumule. The occurrence of a slit in the mature plumule-sheath is 
in all probability caused by the incomplete junction of the margins of 
these collar-shaped projections. The tissue above this second depres- 
sion, which sometimes curves down to a considerable degree pro- 
tecting the plumule, forms that part of the cotyledon known as the 
cotyledonary-sheath. Thus the origin of the plumule sheath indicates 
that it must bea part of the scutellum. To regard the plumule-sheath — 
as an independent leaf as Bruns has done, one must look upon it as 
arising from a stem and not from a primordial leaf, which, according t0 
Hanstein’s investigations, is evidently the case, 
