EXPEDITIOUS TRANSLOCATION 281 



promptly removed. It is, therefore, to be expected that plants should 

 have evolved arrangements for the most direct and expeditious removal 

 of the products of photosynthesis from the green cells ; it follows, as a 

 corollary, that the utilisation of one and the same tissue for transloca- 

 tion and photosynthesis must be avoided as far as possible. In an 

 ideal photosynthetic system every green cell would be in a position to 

 pass on its synthetic products at once to some member of a different 

 tissue specialised for translocation. 



The course followed by the outgoing current of synthetic pro- 

 ducts in any particular case depends upon the extent to which this 

 principle of expeditious translocation is carried out. When this 

 principle is ignored altogether, the photosynthetic cells themselves 

 serve for translocation, and the direction of the efferent stream is 

 longitudinal over the whole of its course from apex to base of the 

 leaf. In more highly organised leaves, the stream at first either flows 

 sideways in the leaf, thus taking the shortest path to the longitudinal 

 vascular strands, or else passes vertically inwards towards a dense 

 network of larger and smaller vascular bundles which extends in every 

 direction beneath the photosynthetic tissue. 



The flanges or partitions evolved in accordance with the principle of 

 maximum exposure of surface, arrange themselves in relation to the 

 lines of flow of the translocatory stream ; hence the elongated tubular 

 form affected by the majority of photosynthetic cells, and particularly 

 by the palisade-units. 



1. Various Modes of Construction of the Photosynthetic System. 



It has already been explained that the great diversity displayed by 

 the photosynthetic system in respect of the details of its construction 

 a diversity which will presently be illustrated by a number of 

 concrete examples is clue to the fact that the principle of expeditious 

 translocation is itself carried out with a varying degree of thoroughness. 

 The author has accordingly classified the various modes of construction 

 of the photosynthetic system with reference to ten leading types, which 

 in their turn may all be relegated to one or other of three principal 

 svstems of construction. 



First System. The simplest type of photosynthetic system is charac- 

 terised by the fact that the photosynthetic tissue is itself responsible 

 for the removal of synthetic products from the entire organ. As the 

 partitions are accordingly placed parallel to the long axis of the organ, 

 the photosynthetic cells are elongated in the same sense, and lie in a 

 plane parallel to the surface. These cells form a single layer in the 

 leaves of most Mosses, while two layers are present in Elodea canadensis ; 

 in Galanthus, Leucojum and Sempervivum the corresponding tissue 



