74 



COMPOUND ORGANS OP PLANTS. 



culi or costae which proceed from the sides of the midrib, each 

 fasciculus consisting of woody fibre enclosing spiral vessels 

 throughout all its ramifications. 



The cellular system of the leaf. This substance forms its 

 principal part, filling up the meshes in the network, formed by 

 the vascular system. To the naked eye it appears as a struc- 

 tureless pulpy mass of a green color, called parenchyma (rfapa, 

 beside or between, and gcv/ta, anything effused or spread out.) 

 Under the microscope the parenchyma of the leaf no longer 

 appears as an unformed mass, but as a beautiful and regular 

 arrangement of cells, which are so disposed as most effectively 

 to subserve those purposes of nutrition for which the leaf is 

 formed. 



In all leaves which present one surface to the sky and the 

 other to the ground, there is between the upper and under 

 cuticle two strata of parenchyma differently arranged. In the 

 upper stratum of parenchyma, the cells are arranged in one or 

 more compact layers, vertically, or at right angles to the upper 

 surface of the leaf, so that they present the least possible 



Fig. 13. 



Fig. 13. Magnified view of the edge of a leaf. The parenchyma is alone represented, 

 the woody tissue being left out. a and b, show the epidermis and denser parenchyma 

 of the upper surface of the leaf; c, d, the looser parenchyma and epidermis of its 

 lower surface. 



