SECT. II PHYSIOLOGY 273 



during active growth. Thus, for example, the rate of growth changes 

 with the alternation of day and night. This will be readily understood 

 from what has been said previously. AVhat is much more striking is 

 that the periodicity continues where the external factors have become 

 uniform. This is termed an after effect of the cause and we shall meet 

 with it in other cases also. Another example of this is the behaviour 

 of our native trees when transplanted to Madeira ; they maintain their 

 old periodicity although they could grow continuously throughout the 

 year. 



Besides the daily periodicity in growth in length, jjeriodic modifica- 

 tions in the forms of stem and leaf are met with, which are not only 

 quantitative but qualitative ; foliage leaves alternate with scale-leaves 

 or bracts, leafy shoots alternate with rhizomes, etc. In these phenomena 

 we have again to do with correlation of growth which was considered 

 on p. 254. The production of a certain quantity of foliage exerts 

 an influence on the further rudiments arising at the growing jDoint 

 and causes them to develop into scale-leaves, bud-scales, etc. If the 

 foliage leaves are removed in the early summer, foliage leaves instead 

 of scale-leaves are formed from the rudimentary leaves at the apex. 

 In a corresponding way the removal of the leafy shoots frequently 

 acts on the subterranean rhizome and causes it to grow above ground 

 and to form foliage leaves instead of scale-leaves ("^). The existence 

 or rather the functioning of one organ thus forces another which is 

 potentially similar to develop in a different way. 



Lastly, we have to consider the periodic alternation expressed in 

 the duration of life of the plant, as a whole. There are plants, such 

 as Stellaria media and Senecio vulgaris, which in a few weeks go through 

 their whole development from the germination of the seed to the ripen- 

 ing of their seeds. Since each seed can germinate at once, several 

 generations may be developed within the year. The individual plant 

 dies on producing a certain number of seeds, but the seeds ensure the 

 maintenance of the type of plant. Many annual plants are similar 

 though their life is more closely connected with the seasons of the 

 year. With these may be placed other plants which only fruit once 

 (monocarpic) but in which seed-formation is preceded by two or 

 many years of purely vegetative growth, with or without resting periods. 

 Probably in all these cases the development of fruit is the cause of the 

 death of the vegetative organs, for their life can be considerably pro- 

 longed by preventing seed-formation. In contrast to these plants, 

 others, such as our native trees, fruit repeatedly, the existence of 

 the individual not being terminated by seed-formation. All perennial 

 types exhibit another periodicity besides that due to the seasons. 

 A tree in its first year when it is a seedling has less intensity of 

 growth than many annual plants ; the intensity of growth increases 

 gradually and its growth in length, its growth in thickness, and even 

 the elementary organs of the wood continue to increase in size until 



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