470 
Wager . — The Perception of Light in Plants. 
In all these cases, therefore, both Haberlandt’s hypothesis, and the 
variation of it which I have brought forward above, fail to account for 
the orientation of the leaf. 
Ortho-heliotropic Organs. 
The epidermal cells of many ortho-heliotropically sensitive organs 
show a pronounced optical activity. They are usually elongate, more or 
less cylindrical cells, and in consequence bring a beam of light to a focal 
line instead of a focal point. It will be convenient to speak of them as 
astigmatic lens cells. They are, however, probably not functional in light 
perception. Some of the most sensitive heliotropic organs do not possess 
them. They frequently occur on leaves which do not show any heliotropic 
sensitiveness ; in some cases such leaves even possess clearly marked and 
optically active papillae or lens-shaped thickenings of the cuticle. 
C. and F. Darwin point out (loc. cit.) that the exciting cause of the 
light response in positively heliotropic organs, such as seedlings, is the 
difference in illumination of the two sides. Haberlandt also considers 
that this is sufficient to account for the stimulus, and that in consequence 
no special light-perceiving cells or cell groups are necessary. The observa- 
tions which I have made, although they show that optically active epidermal 
cells frequently occur, tend to confirm this view. 
In grasses the leaf-blade shows very little if any sensitiveness to 
light, but, as is well known, the young seedlings are strikingly heliotropic. 
According to Figdor 1 the first sheathing leaf or cotyledon and the basal 
sheathing portions of the leaves are entirely responsible for the response 
observed in grass seedlings. The epidermal cells on both sides of the leaf- 
blade of Arena sativa and the inner epidermal cells of the cotyledon exhibit 
a very pronounced and beautiful astigmatic convergence (Fig. 22) ; the outer 
walls at the ends of the cells are slightly more curved, and are capable of 
forming clear images, such as those of a pipe, shown in the figure. The 
epidermal cells on the outside of the cotyledon, which are the only ones 
exposed to the light, are, on the other hand, optically inactive. 
The aphotometric leaves of Glyceria fluitans possess well-marked 
papillate upper epidermal cells. These papillae occur near the end of 
each cell and produce very clear spots of light on the basal walls. Their 
function is, however, to prevent the wetting of the leaf and apparently has 
nothing to do with heliotropic activity. 
The cells of both the upper and lower epidermis of Deschampsia 
caespitosa show a well-marked convergence due to long astigmatic lens 
cells. The leaves are aphotometric : they spread out and bend downwards 
1 Ueber Heliotropismus und Geotropismus der Gramineenblatter. Ber. d. d. bot. Gesell., xxiii, 
I 9°5* 
