282 PHOTOSYNTHETIC SYSTEM 



consists of several layers, and is sometimes quite massive. In the case 

 of Galanthus nivalis the adaxial half of the leaf comprises, in addition 

 to the epidermis, three to four layers of cells provided with numerous 

 chloroplasts. These elements are extended longitudinally, and form 

 very regular series running along the leaf parallel to its surface ; the 

 individual cells vary greatly in length, but are, as a rule, from two to 

 five times longer than their width. The lower half of the mesophyll 

 likewise consists of rows of longitudinally extended cells, forming a 

 photosynthetic layer which is nearly as thick as that in the adaxial 

 half; this abaxial tissue is, however, less richly provided with 

 chlorophyll, and is, on the other hand, traversed by more numerous 

 intercellular spaces. Some of the plants exemplifying this first 

 system exhibit the beginnings of a division of labour between photo- 

 synthetic and translocatory cells. In Elodca canadensis, for example, 



those cells in the lower of the two com- 

 ponent layers of the leaf which contain 

 least chlorophyll are longer and narrower, 

 and are thus more especially adapted for 

 purposes of translocation. 



Second System. At a somewhat higher 

 surface section (Tangential l.s.) level of organisation, distinct tissues are 

 *? %J&&%?Xf regularly set apart for photosynthesis and 



for translocation, the synthetic products 

 being transferred directly from one to the other. The types in which 

 this plan is carried out, display the greatest variety as regards minor 

 features ; there are, in fact, innumerable modifications differing in points 

 of detail, but nevertheless all evidently conforming to the two aforesaid 

 guiding principles. 



The simplest variety of the second system is exemplified by the 

 genera Gladiolus and Tritonia, and also by Iris germanica ; it is 

 characterised by the combination of longitudinal translocatory channels 

 with transversely extended photosynthetic cells. Gladiolus illustrates 

 this variety in its most highly developed form (Fig. 112). Here the 

 transverse diameter of the photosynthetic cells exceeds the length of 

 their (morphological) longitudinal axis from four- to seven-fold. As seen 

 in a transverse section of the leaf, these cells appear to be in almost 

 uninterrupted contact with one another ; a surface (tangential) section, 

 however, reveals the presence of fairly extensive intercellular spaces 

 between their transverse walls. These spaces act as transverse barriers 

 which prevent the synthetic products from travelling at right angles to 

 the long axis of the cells. . The outgoing stream of plastic material is 

 thus forced to travel across the leaf towards the proper channels of 

 translocation, which are represented partly by the parenchymatous 



