liO SPONGY TISSUE 



is protected from the injurious effects of intense illumination. 

 Owing to the considerable length of the palisade cells, there is 

 accommodation for a large number of chloroplasts in each. In 

 some plants, however, the chloroplast-bearing surface is increased 

 by the development of special infoldings of the wall [arm-palisade , 

 e.g. in the leaves of the Elder (Fig. 53, a.p.) and White Lily]. 



In the transverse section of the leaf of the Fuchsia two or 

 three palisade cells frequently join by their lower ends on to one 

 and the same cell of the spongy parenchyma (Fig. 51 ; see also 

 Fig. 53, c.c). The cells in question are usually broadened at 

 their upper ends, so that they are more or less funnel-shaped. 

 Such cells are usually observed in leaves having the kind of 

 structure here described ; in fact more than three palisade cells 

 may be found thus connected with one spongy cell {e.g. India- 

 rubber plant, i^j'cMS elastica ; Oleander, Nerimn oleander, Fig. 82, 

 Col). It is probable that the assimilation-products (carbo- 

 hydrates, etc.) formed in the palisade cells pass into these collecting 

 cells and from them diffuse, via other spongy elements, to the 

 veins. 



The spongy parenchyma, in its most typical form {e.g. in 

 Euphorbia amygdaloides), consists of irregularly lobed cells 

 attached to one another by their projecting arms (cf. Fig. 82), 

 so that wide intercellular spaces occur between them. In the 

 leaf of the Fuchsia and in many other cases, however, the cells 

 are more rounded and the interspaces consequently smaller (Fig. 

 51, In. ; Fig. 52, B). The rather few chloroplasts in the spongy, 

 as compared with the palisade, parenchyma may be related to 

 the fact that the former tissue receives relatively little light. 

 The layer in contact with the lower epidermis is not uncommonly 

 continuous (except for the gaps constituted by the often large 

 respiratory cavities beneath the stomata. Fig. 51, R.), and its 

 cells may even show a palisade-like form ; in this case they 

 generally contain rather numerous chloroplasts {e.g. Fuchsia, 

 Corn Cockle), and serve to utilise the light reaching the 

 under-side of the leaf. 



The chief function of the spongy tissue is to constitute an 

 extensive intercellular system communicating on the one hand 

 with the atmosphere by way of the stomata, and on the other 

 hand with the entire aerating system of the rest of the plant. 



