194 PLANT PHYSIOLOGY 



ments, the cambium and its daughter cells. According to 

 the demand, the stimulus, thus exerted, these daughter cells 

 develop into the so-called "spring wood." 



The hypothesis thus outlined would be worthless if it were 

 not for the early growth of the roots which makes possible 

 the supply of the relatively large volume of water demanded 

 by the parts coming from the bud. Goff* has recently 

 shown that growth of the root begins in the spring before 

 there are any signs of growth in the parts above ground. 

 Thus the plant is early provided with the absorbing agent 

 needed. This growth of the root also must be regarded as 

 the irritable response to the stimulus exerted upon it by the 

 moisture and the increasing warmth of the soil. 



The other half of the annual ring remains to be accounted 

 for. What has already been said regarding the effect of in- 

 creasing the mechanical strains to which growing parts are 

 subjected (see pp. 174, 187-8) prepares us for an hypothe- 

 sis, deserving more experimental tests, to account for the 

 change in the character of the season's growth of wood. 

 After the buds open, the leaves expand and grow, the inter- 

 nodes lengthen, and all the parts and their component cells 

 attain their definitive dimensions, weights, etc. As a result, 

 the mechanical strain upon the parts behind increases. It 

 increases not only with the weight and with the change in 

 position of the weight, which produces a greater leverage, 

 but also with occasional sudden and often very great addi- 

 tions to the weight by wind and rain. Meantime there is 

 little or no increased demand for food, and transpiration, 

 being controllable by the stomata, is not likely to increase 

 greatly. There is ordinarily, therefore, no great addition to 

 the conducting system (see pp. 123-4). 



Strengthening tissues fibres, tracheids, thick-walled ele- 

 ments are formed by the differentiation of the young cells 

 derived from the cambium. From Hegler's investigations 

 (p. 187 ) it is evident that strengthening tissues develop ac- 

 cording to the strain to which a part is subjected, that an 

 increasing strain is accompanied by the formation of more 



* Goff, E. S. The resumption of root-growth in spring. Wisconsin 

 Agric. Exp. Sta.. 15th Annual Report 1898. 



