288 



PHOTOSYNTHETIC SYSTEM 



supply-pipes, through which the synthetic products are conveyed from 

 the palisade-tissue to the ultimate branches of the vascular reticulum. 

 Incidentally it may be noted that the spongy parenchyma also contains 

 some chlorophyll, though not a great amount, and is thus capable of 

 photosynthetic activity in a minor degree, while it also acts as the 

 principal ventilating tissue of the leaf by virtue of the numerous air- 

 spaces with which it is provided. The spongy parenchyma, therefore, 

 illustrates the unusual phenomenon of a tissue which is simultaneously 

 adapted in relation to several distinct functions [of approximately 

 equal importance]. 



The intermediary tissue occupies the larger and smaller meshes of 

 the network of efferent strands composed of the parenchymatous 



Fig. 118. 



i 



T.S. through the leaf of Raphanus sativus, including a secondary vein and the 



adjoining mesophyll. 



sheaths that surround the vascular bundles. As far as the finer 

 branches of the vascular system are concerned, these conducting 

 parenuiyma-sheaths [border parenchyma] 140a consist of a single layer 

 of more or less elongated cells containing but little chlorophyll. Not 

 infrequently the cells of the sheath are provided with lateral extensions, 

 in order to facilitate communication with the spongy tissue ; properly 

 speaking, these lateral branches belong to the efferent system. In 

 passing from smaller to larger vascular strands, one finds that the 

 simple bundle-sheath becomes replaced by a many-layered structure 

 which in its turn gradually gives way to the more massive and less 

 individualised "nerve-parenchyma" (Fig. 118) that forms the great 

 bulk of the mid-rib and the principal lateral veins. This nerve- 

 parenchyma is continuous with the parenchymatous ground-tissue of 

 the petiole and stem. It is thus quite evident that the enlargement 

 of the cross-sectional area of the conducing -' " 



