SE °r- n PHYSIOLOGY 251 



THE RESTING PORTIONS OF THE PROTOPLASM, THAT IS, BY THE SURFACE 



layer. 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 (and electricity), heat, gravity, chemical influ- 

 ences (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, a stimulus received by a 

 non-motile organ may be conveyed to an organ capable of motion, and there produce 

 movement. In the case of roots, for example, the geotropic stimulus is received 

 by the non-motile root-tip, while the movement is induced in the part of the root 

 in process of elongation. 



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

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

 particular inciting stimulus, as heliotropism, geotropism, hydrotropism, 

 etc. ; and these again are either positive or negative, according as the 

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

 stimulus ; while 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. Dorsiventral 

 organs, in particular, exhibit a tendency to assume diatropic and even 

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

 flowers usually fall off; 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. 

 Upon them the light has a most injurious, even fatal, effect, as may be 

 easily observed in the case of Fungi and Bacteria. The hygienic 

 importance of daylight in dwelling-places is due to the destructive 

 action of light upon such forms of plant life. That some plants seek 

 the light, while others avoid it, is not surprising in view of the 

 adaptability which organisms usually exhibit in respect to the in- 

 fluences with which they come in contact in the natural course of their 

 development. 



A good opportunity for the observation of heliotropic phenomena 



