THIRD TYPE OF CONSTRUCTION 



289 



hand with the increase in the amount of plastic material that has to 

 be transported ; a similar spectacle is afforded by a complicated river- 

 system, with its main channel and countless tributaries. 



The several types which conform to this third system of construc- 

 tion, differ from one another mainly as regards the relative degree of 

 development of the component tissues. Thus palisade-tissue may 

 preponderate over spongy parenchyma and vice versa ; even different 

 leaves of the same species may differ appreciably in this respect. The 

 causes of such variation will be discussed in a subsequent section of 

 the present chapter (Section IV.). An excellent illustration of 

 complex organisation, combined with high physiological efficiency, is 



Fie. 119. 



Part of a T.S. through the leaf of Ficus elastica. p, p u the two adaxial palisade- 

 layers ; a, collecting-cells ; </, a small vascular strand composed entirely of tracheides ; 

 s, parenchymatous sheath of the strand, x 230. 



provided by the leaf of Ficus elastica (described in detail in the author's 

 V crgleichende Anatomic des Assimilationssy stems). The mesophyll of 

 this leaf comprises, apart from the vascular bundles and their sheaths 

 (Fig. 119) : (a) two palisade-layers ; (b) from six to ten layers of spongy 

 tissue ; (c) a layer of funnel-shaped collecting cells interposed between 

 (a) and (b) ; (d) a layer of cells, situated immediately within the lower 

 epidermis, which, though shorter than the palisade-cells, approach the 

 latter in general form. The presence of the last-mentioned layer 

 illustrates a tendency on the part of the plant to extend the photo- 

 synthetic system beyond its ordinary limits, wherever illumination 

 provides an opportunity. Such an abaxial palisade-layer recurs in 

 ', various other plants ; it is generally composed of funnel- or hour-glass- 

 V shaped cells, and constitutes, as it were, a miniature copy of the principal 

 adaxial system. The opposite extreme as regards the organisation of 



T 





