EFFECTS OF GROWTH AS SEEN IN MOVEMENTS OF PLANT-ORGANS. 9 



THE EFFECTS OF GROWTH AS SEEN IN CERTAIN 

 MOVEMENTS OF PLANT-ORGANS. 



By Rev. Professok G. Henslow, M.A., V.M.H., &c. 



[Lecture delivered April 6, 1909.] . 



Introduction. — One of the most prominent effects of growth is the result- 

 ing movements of plant-organs. Though the most obvious characteristic 

 of plants in general is that they are fixtures in the soil ; yet, probably all 

 the higher plants, at least, have their growing parts more or less in 

 motion for a time. In some cases of microscopic aquatic organisms, the 

 whole being is in motion in the water, as, e.g., diatoms, Oscillatorias, and 

 the zoospores of Algae. It is not known for certain how the first two * 

 effect their movements, but they are in some way dependent on the 

 properties of protoplasm ; the third are provided with protoplasmic cilia, 

 and these are obviously the instruments of motion, 



There are also numerous movements of protoplasm within cells, 

 especially when the process of cell-division takes place. The well known 

 "streaming " and "rotation " are continually going on and well seen in 

 Chara ; but these will not concern us now. 



Other movements of organs are also the result of forces within the 

 cells, and not the direct consequence of external agencies ; some of these 

 will be referred to ; but it is especially the movements resulting from 

 stimuli of the surrounding physical conditions of life which I propose 

 to deal with. 



As soon as plants begin to .grow, their organs are subjected to, and 

 respond to, various external stimuli ; and the problem is to detect which 

 is, or are — as there may be more than one — acting upon any organ at 

 any one time. The chief influences are (1) light and (2) shade ; 

 i.e., really, varying degrees of light till perfect darkness may occur. 

 (3) Varying degrees of heat and cold. (4) Moisture in the air or soil. 

 (5) Gravitation, always acting in a direction perpendicular to the earth's 

 surface. (6) Contact with a resisting surface. (7) To the preceding may 

 be added the internal influence of cellular growth, which produces external 

 movements of growing shoots. 



Organs may be developed under these influences without movement ; 

 but if their positions be altered, they will move or turn towards the source 

 of the influence. 



Both the tendency to respond to any of these influences may become 

 a fixed and hereditary character, and take place when the original exciting 

 influence is absent ; and the structure itself caused by response may be, 

 and usually is, hereditary as well ; so that it may be formed completely 

 or partially previous to the commencement of the action of the stimulus 

 which had originally given rise to it. 



* See Cryptogamic Botany, by Bennett and Murray, pp. 422 and 442. 



