LEAVITT: TRICHOMES OF ROOT. 
303 
0.6 mm. from the apex. The mother cells were strongly convex, and 
the very short segments, cut from their extremities, were therefore 
at the beginning considerably inferior in breadth and height as well 
as in length, to their sister segments, the future elongated cells of 
the layer. The mature exoderm is shown in longisection in figure 
115 (plate 19). Two short cells sometimes occur in succession. I 
have seen an instance in which this arrangement had just been 
established by the cutting off of a second segment taken from the 
proximal extremity of the longer daughter cell. 
The genetic and structural resemblance of hypodermal short cells 
to trichoblasts is evident. The points of similarity in brief are these. 
Both short cells and trichoblasts arise as very short, often wedge- 
shaped segments. Both are often distinguished throughout life by 
relatively abundant granular protoplasm. De Bary says that the 
nucleus is often relatively large in the short hypodermal cells, as it 
often is in trichoblasts. Both hypodermal short cells and tricho¬ 
blasts are apparently called into existence in response to stimulus. 
The production of transfusion cells is not a constant structural 
feature of the hypodermal layer, at least in many plants, in which I 
have noted that the formation of these cells may be intermitted on 
occasion, even over considerable areas of the hypoderm. In these 
cases it would seem that transfusion cells are formed in response to 
certain environmental conditions — a behavior typical of trichoblastic 
formation. This facultative character is apparent in many Liliiflorae. 
In the epiphytic Aroideae and Orchidaceae. the alternating structure 
has become more fixed. 
There is also a functional similarity. Trichoblasts, or the tri- 
chomes into which they develop, are absorptive cells. The short 
cells of the exoderm have been termed transfusion cells, with 
probable reason. In many cases the arrangement or structure of 
neighboring elements — as for instance the pitted walls of overlying 
velamen cells ( Taeniopliyllum Zollingeri according to Mliller) — 
and the thin membranes of the short cells, point to the short 
cells as gates in the hypodermal protective sheath, or integument. 
Muller says that in older, non-absorptive parts of the roots of Tae- 
niophyllum Zollingeri , the short cells are stopped by the pressing 
down of overlying cells, the walls of which become thickened. 1 
1 Muller, J. : Ueber die anatoinie der assimilationswurzeln von Taenio- 
phyllum Zollingeri. Sitzungsb. k. akad. d. wiss., Wien, math, naturw. cl., 
109 , pt. 1 (1900). 
