Epidermis of Mesemb ry anthemum pseudotruncatellum . 1143 
of the encrustation of the upper epidermis, but showed no sign of papilla¬ 
like structure. The wavy outline of the epidermal cell-walls was exhibited 
to a less extent and the hexagonal shape more approximated to. 
The question of the function of papilla-like cells in the epidermis of 
leaves has been discussed by Haberlandt ( 5 ), Albrecht (1), Kniep (6), 
Stahl (10), Nordhausen ( 9 ), and Wager ( 12 ). Other workers have con¬ 
tributed to the discussion, which has resulted in a considerably less part 
being assigned to these cells in the carrying out of heliotropic responses 
than was formerly the case. 
Very few cases of aphotometric species have, however, been examined 
as to the occurrence thereon of lens-cells. 
Wager has described the efficient lens-cells which are developed in the 
stem and leaf of Mesembryanthemum cordifolimn . Though here local lens- 
cells are present in equal numbers on both sides of the leaves, these 
* always place themselves with the upper surface exposed to, and more or 
less at right angles to, the rays of light * (12). 
In M. pseudotruncatellum we have a plant which in its native habitat is 
subject to uniform, strong, solar illumination. The illumination of the 
upper surface is of this character, and is, in addition, direct. The illumina¬ 
tion of the side cells is also uniform, but diffuse and weaker. The latter 
cells are better qualified as perceptive organs than those of the upper 
surface, and experiments were made to see whether the plant as a whole 
would show any heliotropic response when subject to strong one-sided 
illumination. Plants of different ages, between one and three years, were 
brought into the heliotropic chamber in succession and placed so that the 
middle of the plant experimented upon came opposite the window of the 
chamber. The experiments were performed in a hothouse, where the helio¬ 
tropic chamber was placed with its window facing south. Control plants of 
different ages were also used. Plants were placed alternately with the 
surface slit directed towards the window and at right angles to this direction. 
During three weeks not the slightest sign of heliotropic response was 
seen. 
In a similar experiment seedlings ten days old showed a decided 
curvature towards the window after four hours of direct side illumination. 
On the third day they were all strongly curved, with their upper surfaces 
inclined almost perpendicularly to the incident rays (Figs. 9 and 10). 
At 11 a.m. on the third day the seedlings were turned through an 
angle of 180 0 . By 4 p.m. they had lost the greater part of their curvature, 
while the upper surface had become horizontal. By noon of the following 
day they had all curved strongly in the direction of the light. As men¬ 
tioned above, seedlings of this age possess no lens-cells. 
From the age of ten days onwards the seedlings continued to exhibit 
heliotropic curvatures under illumination from the side. Seedlings forty 
