282 



PLANT STRUCTURES 



150. Mestome and stereome. — When the plant body be< 

 comes complex a conductive system is necessary, so that 

 the different regions of the body may be put into communi- 

 cation. The material absorbed 

 by the roots must be carried to 

 the leaves, and the food manu- 

 factured in the leaves must 

 be carried to regions of growth 

 and storage. This business of 

 transportation is provided for 

 by the specially organized ves- 

 sels referred to in preceding 

 chapters, and all conducting tis- 

 sue, of whatever kind, is spoken 

 of collectively as mestome. 



If a complex body is to main- 

 tain its form, and especially if 

 it is to stand upright and be- 

 come large, it must develop 

 structures rigid enough to fur- 

 nish mechanical support. All 

 the tissues which serve this pur- 

 pose are collectively known as 

 stereome. 



The sporophyte body of 



Pteridophytes and Spermato- 



phytes, therefore, is mostly 



made up of living and working parenchyma, which is 



traversed by mechanical mestome and stereome. 



151. Dicotyl and Conifer stems. — The stems of these two 

 groups are so nearly alike in general plan that they may 

 be considered together. In fact, the resemblances were 

 once thought to be so important that these two groups 

 were put together and kept distinct from Monocotyledons ; 

 but this was before the gametophyte structures were 

 known to bear very different testimony. 



Fig. 265. Same tissues as in pre- 

 ceding figure, in longitudinal sec- 

 tion, the parenchyma showing 

 nuclei.— Chamberlain. 



