276 SCIENCE PROGRESS 



As the result -of detailed anatomical studies Strasburger was 

 led to consider the mesophyll of the leaf, together with the 

 bundle sheath, as homologous with the whole of the tissue 

 outside the vascular bundle proper in the petiole, etc. He 

 would attribute to the bundle a conducting function and to the 

 tissue outside it primarily an assimilatory function. This view 

 seems a very natural one when the course of evolution of plant 

 form is considered. In the lowest forms assimilation is carried 

 out by the whole of the external surface, but in ascending the 

 scale it is seen that there is a tendency to differentiate the plant 

 body and to form, among other organs, leaves or localised 

 areas of tissue speciall}^ fitted for carrying out photosynthesis. 

 Together with this has gone the evolution of a special conduct- 

 ing tissue — the vascular bundle 



Other parts of the plant supply supporting and storage 

 tissues and to this category the cortex of herbaceous plants 

 may be referred, though in addition it very frequently contains 

 chlorophyll and can form sugars and starch. Similarly the homo- 

 logous nerve parenchyma is primarily a tissue for the support of 

 the assimilatory cells of the leaves. In the case of larger leaves 

 it may also serve for the temporary storage of such reserves as 

 may diffuse into it. A certain amount of the palisade and 

 spongy mesophyll abuts directly on the nerve parenchyma in 

 most cases and diffusion of sugars goes on through the lateral 

 walls of its cells. Moreover there is nothing to prevent 

 diffusion from cell to cell in the longitudinal direction. The 

 main mass of the sugar, however, undoubtedly passes from 

 the spongy mesophyll into the single-layered sheaths of the 

 innumerable fine veins forming a close meshwork throughout 

 the lamina. 



These single-layered sheaths provide a tissue for the recep- 

 tion of the assimilates passed in from the surrounding cells and 

 for their subsequent transference into the phloem of the vascular 

 bundle, for translocation in the sieve-tubes. The structure of 

 the small sheaths is in every way admirably adapted for this 

 function. The absence of intercellular spaces between its inner 

 walls and the cells of the bundle ensures that the largest pos- 

 sible area of surface is secured through which the transference 

 may go on in the radial direction. The close approximation 

 of the radial walls facilitates diffusion through them in the 

 tangential direction, as may be necessary to reach the phloem. 



