164 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[32. 



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. 



In a few cases (e.g. Allium ursinum, Alstrcemeria, etc.) the leaf is twisted, so 

 that it is the morphologically lower (dorsal) surface which is directly exposed 

 to the light ; in these cases the palisade-tissue is developed in relation with 

 that surface, the spongy tissue in relation with the morphologically upper 

 (ventral), but shaded, surface. 



When it so happens that all sides of the leaf are equally exposed to light, 

 the palisade-parenchyma 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 dorsi ventral leaves (e.g. leaf -blade of Anchusa italica, Linosyris 

 inilgaris, Silene inflata, Dianthus Caryoplvyllus, 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 spongy 

 parenchyma, but consists of rounded cells (e.g. suc- 

 culent leaves, such as those of species of Crassula, 

 etc.). 



In those leaves in which the external portion only 

 of the mesophyll consists of assimilatory tissue, the 

 internal mesophyll being destitute of chloroplastids, 

 the assimilatory tissue may consist either of palisade- 

 tissue (e.g. some Myrtaceae and Proteactae, species 

 of Statice, Welwitscbia, etc.), or of rounded cells 

 (e.q. Mesembryanthemum, many Monocotyledons). 



The cells of the assimilatory tissue sometimes 

 present other forms and arrangements. Thus the 

 assimilatory tissue of the leaf of Pinus and Cedrus 

 consists of polyhedral cells, the walls of which pre- 

 sent iufoldiogs, the effect of which is to increase the 

 surface of the cell-wall. In other cases it consists 

 entirely or in part of elongated cells, resembling 

 palisade-cells, which are arranged with their long 

 axes parallel to the surface, either parallel to the 

 long axis of the leaf (e.g. Galanthus nivalis, Leu- 

 cojuvi vernum) or transversely (e.g. Iris germanica, 

 Erytlironium Deiis-Canis, species of Gladiolus and Tritonia). 



The colourless mesophyll of succulent and coriaceous leaves consists of large 

 cells, containing much watery sap, constituting in fact (see p. 161) an aqueous 

 tissue (e.g. leaves of Aloe, Mesembryanthemum, some Myrtaceae and Proteaceae, 

 Welwitschia, etc.). In some Orchids (e.g. Oncidium maximum), the cells of the 

 aqueous tissue are scattered among the assimilatory cells ; in many Orchids 

 the cells of the aqueous tissue are tracheidal, having spirally-thickened walls, 

 as they are also in the stem and leaf of Nepentbes. 



In many cases, especially in aquatic plants, the ground- tissue has 



FIG. 126. Diagrammatic 

 transverse section of the 

 acicular leaf of a Fir: e 

 epidermis; es sclerenchy- 

 matous hypoderma ; sp sto- 

 mata; h resin-ducts; s cn- 

 dodermis enclosing the 

 tingle meristele ; g wood ; b 

 bast. 





