THE MECHANISM AND CAUSES OF TRANSLOCATION 591 



once emptied appears to have fulfilled its purpose and to have lost the 

 power of renewed storage (Puriewitsch, I.e., p. 211). The structure of the 

 conductoiy channels and their connexions with the surrounding tissue are 

 of the utmost importance in translocation, and the point of union of the 

 leaf-trace-bundle will partially determine the length of the course through 

 which the plastic products from the leaf must travel in order to reach 

 the growing apex. The conducting channels may be more or less com- 

 pletely isolated by being surrounded by air spaces or less permeable tissues. 

 Similar means are adopted by the plant to lessen the intercourse with 

 the external world, and the suberized endodermis and similar layers are 

 probably of importance for insulating purposes and to prevent irregular 

 diffusion l . 



Corky and cuticular coverings seem to prevent the extraction of 

 diosmosing food-materials from parts in contact with water, and this is of 

 great importance in the case of beetroots, turnips, bulbs and rhizomes. 

 Puriewitsch found that a bulb-scale of an onion lost no perceptible quantity 

 of sugar while its cuticularized surface was in contact with water, but 

 that the whole of the stored sugar was removed when a cut surface was 

 bathed with renewed supplies of liquid. Cork and cuticle are absent 

 from the young root where a power of rapid exchange is necessary, and 

 sugar is actually excreted from the root of a seedling imbedded in gypsum, 

 in which case plastic materials accumulate to excess owing to the enforced 

 cessation of growth. Over-accumulation is normally prevented by energetic 

 consumption, by translocation, and by temporary storage in the phloem and 

 neighbouring tissues, while in semi-aquatic plants the upwardly directed 

 water-current aids in preventing any loss through the roots. Other factors 

 may also enter into play, and it is possible that mere contact with water is 

 insufficient to withdraw any of the transitorily stored substances from the 

 intact root-apex. Some such specific peculiarity is essential in the case of 

 algae to prevent the escape of the stored materials, for if the thallus of these 

 plants behaved similarly to endosperm-tissues no accumulation of sugar 

 or starch would be possible so long as it was in contact with water, 

 whereas as a matter of fact starch is often present in considerable 

 abundance. Methyl-blue is permanently retained by living cells when 

 once absorbed, and it is only when traces of diosmosing products are 

 continually formed and removed that a non-diosmosing substance gradually 

 disappears from a cell (Pfeffer, I.e., p. 296). Both the outer and inner walls 

 of algal filaments are readily permeable, and hence the diosmotic tranference 

 of sugar from one cell to another would apparently not be possible without 

 great waste. It has yet to be determined by what means this loss is 

 prevented, or whether protoplasmic connexions become in this case of 



1 Cf. Haberlancit, Physiol. Anat., 1896, 2. Aufl., p. 298. 



