274 



SCIENCE PROGRESS 



resembles that of the cells of the small sheaths and Haberlandt 

 and Schubert considered that the nerve parenchyma is to be 

 regarded as an amplified sheath. 



Before dealing with this point the mode in which the 

 small sheaths and the surrounding spongy mesophyll are 

 connected may be noted. When the tissue of the leaf is closely 

 arranged and the amount of spongy mesophyll is small, certain 

 cells of the latter tissue unite with the sheath cells without 

 causing any marked alteration in their shape. On the other 



Fig. II. — Tangential section of leaf vein of RiUa graveolens showing the long 

 projecting arms of the sheath cells. 



Fig. 12. — Transverse section of vein of Rula graveolens. 



Fig. 13.— Longitudinal section of a sheath in Papaver Rhaas. 

 (Figs. 11-13 after Schubert.) 



hand, in leaves with comparatively loosely arranged assimilatory 

 cells, it frequently happens that the connection is effected by 

 means of tubular outgrowths from the sheath cells, which then 

 present a less regular appearance in transverse section. 

 (Figs. 11-13.) 



It was found that the sheaths of the finest veins are provided 

 with more numerous projecting arms than are the sheaths of 

 the larger veins. This was accounted for by supposing that 



