2o6 



LEAF-AKATOMY OF SCiEVOLA CKASSIFOLIA. 



many as five rows of palisade-celis. This central tissue is re- 

 markable in the ver\' old leaves, as its cells become enlarged, 

 lose the greater part or all of their chlorophyll, and take on the 

 function of water-storage. Those cells immediate) v surrounding 

 the vascular strands are the first to assume the new function 



(PI. XX 



tig. 5). From this central, water-storage tissue and 



from the palisade tissue, special water-storage organs, such as 

 mucilage cells and water-storage tracheides are differentiated. 



Miic'daye cells. — lu the early stages of differentiation, certain 

 cells, particularly in the palisade-region, develop mucilaginous 

 walls. These cells keep pace with the growth in thickness of 

 the leaf by branching in all directions, producing, thus, a peculiar 

 and striking result (Text-fig.5). Text-fig. Gti shows one of tliese 

 branched mucilage-cells swollen after treatment with potash, and 

 stained with corallin soda to show successive deposits uf mucilage 

 in the cell-wall. 



a. b 



'IV-xt-H^^.d : rt, single luucila^e-cell cilter tit'atiuent with potash, and stained 

 with curallin-soda, to show strati tication of luuoilage; ( x 230) : /', a 

 group of water- storajfe tracheides from the central tissue of the 

 mature leaf; ( x 400). 



Water star (iqe tracheides. — llestricted to the central tissue are 

 special water-storage tracheides occurring both at the bundle ends, 

 and in groups, quite independent of the bundles. They are of 



