114 



PART II. ANATOMY AND HISTOLOGY. 



[31 



In view of its great physiological importance a somewhat detailed ac- 

 count of the structure of the mesophyll of the leaf-blade seems necessary. 



The mesophyll consists of parenchymatous thin-walled cells of various 

 form. When the blade is thin, the whole mesophyll consists of assimila- 

 tory tissue ; but when it is more or less fleshy and succulent, the more 

 central part consists of cells without chloroplastids, the assimilatory 

 tissue being confined to the surface. 



When the mesophyll is altogether assimilatory, the arrangement of the 

 cells is correlated with the symmetry of the leaf-blade. In a dorsiventral 

 lamina (Fig. 92) the structure of the mesophyll is different an relation 

 with the upper (ventral) and the lower (dorsal) surfaces. Towards the 

 upper surface, Avhich is more directly exposed to light, the somewhat 

 elongated cylindrical cells form a compact palisade-tissue one or more 

 layers in thickness ; whereas, towards the lower shaded surface, the cells 

 are less regular, frequently somewhat stellate in form, leaving lai-ge 

 intercellular spaces between them, constituting what is known as the 

 spongy parenchyma. The loose structure of the 

 mesophyll towards the lower surface of the 

 blade is correlated with the presence of nu- 

 merous stomata in the epidermis of that surface 

 (see p. 108). 



When the palisade-parenchyma consists of 

 several layers, the transition from the one form 

 of tissue to the other is gradual. The vascular 

 bundles run along the junction of the two forms 

 of tissue. 



When it so happens that all sides of the leaf 

 are equally exposed to light, the palisade-paren- 

 chyma is developed in relation with both the 

 dorsal and the ventral surfaces ; this is true, 

 not only of isobilateral and of radial leaves, but 

 also of dorsiventral leaves (e.g. leaf-blade of 

 Anchusa italica, Linosyris vulgaris, Silene inflata, 

 Dianthus Caryophyllus, etc.) ; in which case the 

 spongy parenchyma is either absent, or consists 

 of a few layers in the middle of the blade, but 

 the intercellular spaces between the palisade- 

 cells are, however, relatively large. 

 In some cases, the mesophyll is not differentiated into palisade and 

 sj>ongy parenchyma, but consists of rounded cells (e.g. succulent leaves, 

 such as those species of Crassula, etc.). 



The cells of the assimilatory tissue sometimes present other forms and 

 arrangements. Thus the assimilatory tissue of the leaf of Pimis and 

 Cedrus consists of polyhedral cells, the walls of which present infoldings. 

 the elfect of which is to increase the surface of the cell-wall. In other 

 cam* it consists entirely or in part of elongated cells. reseml>Hng_palisade- 

 ci-lls, which are arranged with their long axes parallel to the surface. 

 either parallel to the long axis of the leaf (e.rj. Galanthus nil-alls, the Snow- 



Fio. 93. Diagrammatic 

 transverse section of the 

 acicular leaf of a Fir : e 

 epidermis ; es sclerenchy- 

 matous hypoderma ; p sto- 

 mata ; /i resin-ducts ; s en- 

 dodermis enclosing the 

 single meristele ; gwood; b 

 bast. 



