GROWTH AND MOVEMENT 



123 



the other in darkness, the formerJwill_[show normal growth with a 

 relatively short and compact form, and with expanded green leaves. 

 The latter will show an abnormally elongated stem, while the leaf- 

 blades are small, and the whole plant will be pale in colour. This 

 condition is styled etiolation, and the length of the stem is due to the 

 absence of the inhibitory influence of light. It has been found that 

 the violet rays are the most effective, and that they have a retarding 

 influence almost equal to that of full sunlight. (Fig. 85.) 



It is the want of sufficient light for the normal development that 

 makes plants in crowded greenhouses, or in dwelling-rooms grow 

 " l^ggy>" with unduly lengthened stems and leaf-stalks. Crowding 

 of field crops has a like effect, and often leads to the " laying " of corn 

 under heavy wind and rain before harvest, the plants being top-heavy 

 and their stems weak. 



\^r. <;<o/*. 



/o. 30 



/O i-a 



Movement in Growing Parts 



Any ordinary normal young shoot appears to be growing slowly but 

 steadily forward. If the growth were equal on all sides the advance 

 would naturally be steady, and 

 in a straight line. Under casual 

 observation this seems to be the 

 fact. But if the growth be 

 analysed by accurate observa- 

 tion it is found to be irregular. 

 From time to time it is greater 

 on one side than on another. 

 The side where the growth is 

 greater becomes convex, and the 

 other concave. The apex will 

 then appear to turn away from 

 the side of greater growth, and 

 a movement will be the result. 

 Such movements though small are 

 the rule in normally growing 

 parts. They were first detected 

 by Darwin in seedlings. By affixing a fine glass bristle, bearing at its 

 end a small bead, upright upon the tip of a growing plant, and from time 

 to time marking the position of the bead upon a horizontal glass plate 

 placed above it, he obtained an enlarged record of the movements of 

 the apex of the growing seedling. They were found to be slow, and 



Fig. 85 bis. 

 Record of circuinnutatiou in a seedling made hy 

 Mr. Malins Smith. Note that the direction was 

 reversed at 8.0 a.m. 



