274 BOTANY PAHT i 



organism is not at present known. In order that an external physical force can 

 operate as a stimulus, there must exist within the living substance definite 

 structures or organs which are influenced by it. Thus, for example, H.\r,r.i:i AMU 

 and NEMEC regard the pressure of starch grains (the position of which in the cell is 

 determined by their weight) on the more or less receptive parts of the limiting 

 layer of the protoplast as the arrangement for perceiving the direction of gravity 

 (statolith theory). The movements of growth occasioned by external stimuli are, 

 for the most part, movements in response to directive stimuli which lead to a 

 definite position of the organ, relatively to the direction of the operative influence. 

 The principal external stimuli that come into consideration are light, heat, gravity, 

 chemical influences (oxygen, nutritive substances, water, etc.), impact and 

 friction. 



As the points of greatest irritability in plants or their organs are often more or 

 less removed from the points where the effect of the stimulation is manifested, a 

 propagation of the stimulation must take place. Thus directive stimuli are 

 received by the apices of organs which do not themselves carry out the movement, 

 but transmit it to the regions in which the movement takes place. In root* tin 

 geotropic stimulus is mainly perceived by the root tip ; in grass-seedlings the 

 stimulus of light is perceived by the tip of the cotyledonary sheath ; in the 

 tentacles of Drosera the contact stimulus affects the glandular swelling at the tip 

 and induces the movement in the portions nearer the base. According to MIEHE 

 even when the growing points are not the only perceptive regions they exercise a 

 controlling influence over the resulting movements of curvature J 80 ). 



The capacity of organs to assume a definite direction by means of 

 curvatures of growth is distinguished, according to the nature of the 

 particular exciting stimulus, as heliotropism, geotropism, hy drotropism, 

 etc. ; and these again are either POSITIVE or NEGATIVE, according as 

 the direction taken by the curvature is towards or away from the 

 source of the stimulus. Plant organs which place themselves more 

 or less transversely to the line of action of the operative forces are 

 termed DIATROPIC. As a special result of diatropism, a transverse 

 position is assumed which is exactly at right angles to the direction 

 in which the influence which acts as the stimulus is exerted. Dorsi- 

 ventral organs, in particular, exhibit a tendency to assume diatropic 

 or transverse positions. 



A. Heliotropism 



The importance of light to plant life is almost incalculable. It is 

 not only absolutely essential for the nutrition of green plants, but it 

 has also a powerful effect upon the growth and general health of the 

 plant. Deprived of light for any length of time, leaves and 

 flowers usually fall oflf; fully developed, vigorous organs of green 

 plants soon become yellow in the dark, and droop and die. Prolonged 

 darkness acts like a poison upon those portions of plants accustomed 

 to the light. On the other hand, exactly the reverse is true of plants 

 or organs whose normal development is accomplished in darkness. 



