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 unper 

 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 VoIvhx. Enf/leiia. etc. In Kiqilena 

 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 gi-ains 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 sen.sory 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. a second. In case of the .sensitive plant the trans- 

 mi.ssion is 30-100 mm. per second. 



Besides the transmission of impulses through the protoplasm 

 of the coll 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 l)een interpreted by .some as the jiathways of the effects 

 of stimulation. There is no central organ of coordination known 



