4 NERVOUS SYSTEMS AND SENSE ORGANS 
arranged in such a way as to favor the reception of light waves. 
This is, of course, not alone for sensation, yet sensation may be 
an important function. Some epidermal cells bulge considerably, 
especially in the velvet-like leaves of tropical forests. Such eleva- 
tions make it possible for the cells to perceive photic stimuli, even 
when their surfaces are wet. Sometimes a whole cell bulges in a 
lens-like manner; sometimes the wall is thickened like a little lens, 
and by these methods the rays of light are brought to a focus upon 
the inner sensitive protoplasm. In many plants the whole upper 
epidermis is developed as a light-perceiving or photic epithelium. 
Also at times the margin or some definite locality has cells espe- 
cially adapted to focus and receive rays of light. Such cells alone 
or in groups are conical with rounded tips, the apex of each has its 
wall thickened or almost biconvex. Such so-called ocelli have been 
proved to condense the light more effectually than the ordinary 
surface cell. 
Stigmata or eye spots are found in certain plant spores and 
among the flagellates, such as Volvux. Huglena. etc. In Euglena 
the light-perceiving ability is confined to sensitive protoplasm near 
the pigment spot. The eye spot or pigment therefore acts as a 
light-screen. 
Geotropic movements of plants are remarkable. The plants of 
high organization especially seem sensitive to the stimuli of gravity. 
Certain cells of roots, stems and leaves are provided with movable 
starch grains. It has been suggested that the movements of these 
starch grains bring about changes for growth and movements ap- 
propriate to the needs of the plants. 
Transmissions of stimuli take place within cells from the points 
stimulated to more distant portions, but they cannot well be deter- 
mined. When the sensory and the reaction organs are more widely 
separated the conduction is more obvious. In plants there are but 
few examples of transmission at a distance, for in many cases of 
marked movements in plants the sensory areas immediately adjoin 
the motor tissue. In other cases the transmission is at a greater 
distance. The velocity of transmission in plants is much lower than 
in animals. Heliotropic and geotropic stimuli are said to require 
five minutes to travel two millimeters, traumic stimuli: 1-2 cm. per 
minute to 1-2 cm. asecond. In case of the sensitive plant the trans- 
mission is 30-100 mm. per second. 
Besides the transmission of impulses through the protoplasm 
of the cell there is the necessity for transmission from cell to cell. 
No special pathways have been clearly determined for the first in 
plants, but protoplasmic threads traverse the whole thickness of the 
cell walls. It is questionable whether there are special structures 
within plant cells for the conduction of stimuli. Strands between 
cells have been interpreted by some as the pathways of the effects 
of stimulation. There is no central organ of coérdination known 
