ORGANIC STRUCTURE. 103 



vessels operate to regulate the expansive force of the 

 distensible membranes of the system under the influ- 

 ence of heat, air, and light. This is not only very 

 visibly the case in twining stems, but also in some 

 others which have aspiring, though not involving, 

 powers. We may instance the larch which, when 

 young and growing rapidly, has not only a wavy* 

 form of stem, which, however, it loses when old, but, 

 when cut up for use, retains this twisted structure so 

 much that it is impossible to keep the scantling in 

 a square or rectangular position, until it has been 

 thoroughly seasoned. 



Another remark may be made relative to climbing 

 perennial stems. They are seen to rise perpendicu- 

 larly so long as they have any thing to cling to ; but 

 when they have surmounted the prop their tendency 

 to rise seems to undergo a change ; because they never 

 afterwards grow with such rapidity either horizon- 

 tally or downward, but become bush-headed. 



Creeping Stems are well exemplified in the 

 strawberry and many species of Graminece. 



Progressive growth. Having noticed the com- 

 ponents of the ligneous stem developed in the first 

 year, by which it gains elevation and increased dia- 

 meter by the inflation of its cellular and vascular 



* The structure of the wood of many trees has a wavy character. 

 This is not only visible in the grain of oak and other indigenous 

 trees, hut it constitutes the principal value of some foreign kinds 

 especially that called satin-wood among cahinet makers. 



