12 PHYSIOLOGY OF NUTRITION. 



assimilation to a very large extent or altogether, and this tissue 

 hence consists chiefly or entirely of palisade cells. 



The possession of palisade parenchyma however is by no means 

 characteristic of the foliage leaves of all plants. If, for example, 

 "we examine transverse sections of the young leaves of Triticum 

 vulgare, we find that the mesophyll, which is bounded by an 

 epidermis both on the upper and the lower surface of the leaf, 

 and is traversed by vascular bundles, is composed throughout of 

 nearly uniform cells, roundish in transverse section. Leaves in 

 which differentiation into palisade and spongy parenchyma does 

 not take place are frequently distinguished by the fact that not 

 the whole of the tissue enclosed by the epidermis consists of 

 chlorophyll-containing cells. If, e.g., we examine a transverse 

 section of a leaf of Iris germanica, we shall find green tissue below 

 the epidermis of the upper and of the under side of the leaf. 

 The vascular bundles stand out distinctly, the bast being covered 

 on the outside by a layer of bast fibres, and we further see at 

 once that the middle layer of the leaf consists of cells full of 

 sap and not green. Such chlorophyll-free cells are also very 

 abundant in the leaves of Hyacinthus orientalis and the succulent 

 leaves of Aloes. 



Reference must here also be made to the interesting fact that 

 leaves with well developed spongy and palisade parenchyma 

 appear for that very reason highly dorsiventral in construction, 

 and rank among extremely plagiotropic organs. But by no means 

 all foliage leaves are strongly dorsiventral in construction ; there 

 are many dicotyledons in which the mesophyll of the leaves is 

 centric in organisation, and these for the most part exhibit a more 

 orthotropic development. Thus, e.g., the leaves of Anchusa italica, 

 Centaurea Jacea, Tragopogon orientalis, Aster Amellus, Genista 

 tinctoria, etc., are centric in structure. In the last the mesophyll 

 consists almost exclusively of cells elongated at right angles to 

 the surface of the leaf. Centaurea Jacea varies considerably, as 

 I have often found, according to its habitat. Individuals well 

 exposed to the sunlight produce long narrow leaves which are 

 comparatively thick. The leaves of shaded plants are thinner, 

 but greater in area. The mesophyll of the leaves of Centaurea 

 Jacea, especially in plants exposed to the sunlight, is not dorsi- 

 ventral bat centric in structure. We easily see, on microscopic 

 examination of delicate transverse sections, that palisade paren- 

 chyma is present both on the upper and under sides of the leaves. 8 



