INTERNAL CORRELATIONS IN PLANTS 117 



grow rapidly and set free multitudes of non-sexual single 

 motile reproductive cells called swimming spores capable 

 of immediate growth into new plants. On the contrary, 

 when water becomes scarce or unsuitable for growth, they 

 form by sexual processes a kind of resting spore which 

 is not motile and is surrounded by a hard thick wall that 

 protects it when the pond dries up. This spore produces 

 a new plant the next season. 



Internal Correlations in Plants are as necessary 

 as their adjustment to the environment. A great many 

 things which are ordinarily taken for granted are in 

 reality the result of accurate correlation. Trees form 

 myriads of leaf buds every year, but only a fraction of 

 them unfold in the spring. If, however, the first crop of 

 leaves is destroyed then another lot of buds unfolds to 

 replace them. Clearly, either the unfolding of the first 

 buds must have acted as a restraining influence on the 

 others, or their absence acts as a stimulus, for the cor- 

 relation is an evident fact. 



It is a commonplace that roots develop at the lower end 

 of cuttings and leaves at the top. Nor does it alter the 

 result if the originally upper part of the cutting be the 

 one stuck in the soil. This is, no doubt, due in large part 

 to the fact that the two ends are subject to different 

 stimuli, but not entirely so, since roots do not develop at 

 all parts which are in contact with the soil and so receive 

 the same stimuli. 



The Mechanism of Tropisms. — The reception of an 

 external stimulus depends on the sensitiveness or irrita- 

 bility of the protoplasm. Whether a response will follow 

 depends on the conduction of this stimulus to some part 

 of the same cell or to other cells at a distance where some 

 other activity of protoplasm can be excited. In the trop- 

 ism of roots and stems the stimulus is received by cells 

 near the tip and some sort of change is propagated 

 through several cells to a region further back where there 

 are induced growth changes which cause the root to bend. 



