ECOLOGY 627 
the blade are numerous, closely set, sessile, glandular hairs 
which, after repeated irritation, secrete an acid digestive juice. 
Each of these, as observed under a microscope in surface view, 
consists of a rosette-arranged set of cells composed of 4 to 12 cell 
radii filled with a crimson-claret pigment. 
Each sensitive hair consists of a somewhat elongated structure 
composed of 3 parts, viz., base, highly sensitive joint and insensi- 
tive shaft. The base consists of large epidermal cells enclosing a 
prolongation of mesophyll cells within, that can receive and 
propagate a stimulus from the sensitive joint to the leaf interior. 
The joint consists of elongated columnar cells that enclose 
similar columnar mesophyll cells with soft, elastic walls. The 
shaft cells are elongated, thick-walled and almost insensitive. 
When a stimulus is applied to any sensitive hair, this affects 
the joint cells and causes upsetting of turgidity and exudation of 
liquids with a consequent contraction of the elastic cells. ‘The 
action is propagated to the midrib region where the cells, by 
contraction along the upper surface and expansion along the 
lower, cause closure of the halves. 
Macfarlane! has shown that when an insect alights on a leaf of 
Dionea, one hair must neither be stimulated twice or 2 hairs on 
the same leaf at slight intervals apart in order to cause closure. 
The caught insect produces a summation of stimuli, and gradual 
tightening of the lamina occurs under the repeated stimuli until 
the halves become closely locked. The glandular hairs now pour 
out an acid digestive secretion which digests the flesh and blood 
of the insect’s body. The soluble nitrogenous substances are 
then absorbed by the glands and assimilated. 
Dr. J. S. Hepburn? found that the secretion of the leaves of 
Dionga contained a protease which was active in the presence 
of 0.2 per cent. hydrochloric acid. This enzyme, therefore, 
resembles pepsin of the gastric juice of man, which also acts in a 
0.2 per cent. hydrochloric acid medium. 
Dr. J. M. Macfarlane, who with Dr. Canby and Charles 
Darwin have separately investigated the structure and phys- 
iological activities of this plant, reports that 2 touches one-fourth 
1 Macfarlane: Contrib. Bot. Lab. U. of Pa., 1: 7-44, 1892. 
? Hepburn: Jour. Franklin Inst., 194: 780, 1922. 
