390 MR. T. W. WOODHEAD ON THE 
frequent in the palisade-layer and large in the spongy parenchyma. 
The chlorophyll granules are less abundant than in the two 
previous forms, and the leaf is distinctly paler, and, as above 
stated, stomata occur abundantly on both surfaces. 
The leaf of the Bilberry is usually described as glabrous, but 
close examination shows that on both surfaces, especially on the 
upper surface, are numerous unicellular hairs with thick and 
warted walls (fig. 59). In addition to these, each tooth of the 
leaf-margin is terminated by a clavate multicellular hair with 
thin walls, and, when young, filled with finely granular contents 
(fig. 60). At the base of each hair ends a fine veinule. In an 
old leat the contents turn brown and the hairs become shrivelled. 
These hairs are usually curved in such a way as to apply their 
apices to the upper surface of the leaf. A few are also found 
on both upper and lower surfaces chiefly over the veins; they 
also occur on the margin and near the base of the slightly 
channelled petiole. The contents are readily plasmolysed by a 
solution of sodium chloride, and they soon regain turgidity in 
water, showing their walls to be permeable. Experiments similar 
to those described by Pfeffer (78), Gregory (39), and others 
suggest them to be capable of absorbing water. Hairs occur on 
the under surface and on the leaf-margin of Vaccinium Vitis- 
idea, and are stated by Lundström (61) and Kerner (52) to be 
absorptive. Lundström described many absorptive hairs, and ina 
list he gives the Bilberry is included. Wille (108) has shown, by 
means of a 1-per-cent. solution of lithium chlorate and subsequent 
spectroscopic examination, that many are capable of absorbing, 
but he sharply criticises Lundstrém’s contention that they are 
“adaptive ” structures; while Gregory (39), Schimper (83), and 
others have given numerous illustrations of hairs functioning 
as absorptive organs, some performing the double function of 
absorption and excretion. Drabble and Lake (24) have recently 
made some useful observations which bear directly on the point 
under consideration. In comparing the strength of the cell- 
sap of the epidermis of several species of plants growing under 
different conditions, they find, for example, that the cell-sap of 
Geranium Robertianum growing under moist conditions has a 
strength equal to a solution of sodium chloride having a concen- 
tration in gram-molecules of +11, while the same species growing 
on rocks under xerophytie conditions had a cell-sap equal to 18. 
