TRANSLOCATION OF CARBOHYDRATES 275 



the cells of the small sheaths serve equally to collect and to 

 conduct the assimilates, whilst those of the larger cells are 

 mainly concerned with conduction. 



The increase in the volume of the sheaths towards the bases 

 of the larger veins was thought to be in accordance with the 

 supposed greater need for conducting tissue to transport the 

 assimilates concentrated in these veins from large areas of 

 the lamina. But it may be pointed out that there is a similar 

 increase in the absolute amount of sieve-tube tissue towards the 

 base of the larger veins, owing to repeated additions from the 

 confluent smaller veins. Thus the relation between the amount 

 of conducting tissue and the amount of conduction holds quite 

 as well in the case of the sieve-tubes as in the case of the bundle 

 sheath and the nerve parenchyma continuous with it. 



Schubert pointed out that the nerve parenchyma has in 

 addition the mechanical function of providing a rigid framework 

 for the leaf. Seen in transverse section its cells are frequently 

 found to form a girder-like tissue and this is so arranged as to 

 resist the bending which the weight of the assimilatory tissues 

 would otherwise cause. Frequently the walls of the outermost 

 layers of its cells are thickened, but undoubtedly the greater 

 part of the support is derived not from this but from the 

 turgidity of the cells, as is indicated by the flagging of leaves 

 when the water supply is insufficient and their rapid recovery 

 after the soil has been watered. 



However Schubert agreed with the theory put forward by 

 Haberlandt in 1882, that the carbohydrates are conducted from 

 the leaf in the parenchymatous bundle sheath and *' its homo- 

 logues in the petiole and stem." 



According to Haberlandt "... the morphological and 

 physiological homologue of the single-layered bundle sheath 

 of the leaf is the whole ground parenchyma of the stem, in 

 which the parenchymatous vascular bundle sheaths sometimes 

 present (* starch sheath ' of Sachs, ' sugar sheath ' of De Vries 

 and * endodermis ' of De Bary) appear as newly differentiated 

 products and so ought not to be homologised with the paren- 

 chymatous sheath of the bundle of the leaf" There certainly is 

 a direct continuity between the single-layered bundle sheath 

 of the veins and the many-layered cortex of the petiole, but the 

 statement that the two are homologous in the morphological 

 sense can scarcely be justified. 



