248 



PLANTS AND MAN 



culent herbaceous species to remain erect provided they have an 

 adequate supply of water. The wilting of many garden annuals 

 during dry periods or after they have been picked is caused by a 

 loss of turgidity resulting from an inadequate 

 water supply or excessive water loss. 



In the case of the large erect body typical 

 of most Pteridophytes and Spermatophytes, 

 sufficient rigidity to support branches and 

 foliage requires more than mere cellulose 

 walls or turgidity. Special supporting tissues 

 become imperative if vegetation is to raise 

 itself any distance above the soil. In plants 

 such tissues can form a fairly rigid framework 

 or skeleton without the articulations and 

 joints characteristic of the motile animal 

 body. The skeleton of a large land plant is 

 developed primarily to withstand the strains 

 of compression and tension to which trunks 

 and branches are continually exposed. Skeletal 

 elements, to a minor extent, are developed in 

 leaves to enable them to maintain their broad 

 surfaces; this is brought about by the net- 

 work of veins with their skeletal tissues. Like- 

 wise in roots, skeletal elements are developed 

 to a lesser extent than in stems; largely for the 

 purpose of withstanding a longitudinal pull. 

 Division of labor among the cells of the plant body — which 

 has resulted in special photosynthetic, conductive and reproduc- 

 tive tissues — has likewise brought about special supporting tissues 

 whose cells function primarily for skeletal purposes. Simplest of 

 all are the tracheids of Gymnosperms, which function both for 

 conduction and support. More specialized are the cells of such 

 supporting tissues as collenchyma and sclerenchyma, highly 

 developed among the Angiosperms. 



Tracheids 



In a typical conifer such as a pine, the bulk of the support 

 afforded by the trunk is in the wood. Conifer wood is made up of 



Fig. 171.— Atra- 

 cheid is an elongated 

 cell with slightly 

 thickened and ligni- 

 fied walls; those of 

 the white pine 

 (illustrated) possess 

 bordered pits in their 

 walls. 



