i68 METABOLISM 



solutions of a substance in two regions shall have different degrees of con- 

 centration. Thus we saw that when the endosperms of grasses were immersed 

 in a large quantity of water the cells were gradually emptied, while a small 

 amount of water rapidly came to contain so much sugar that diffusion could no 

 longer take place. When the migration of the sugar formed from the starch 

 ceases hydrolysis of the starch also comes to an end and the endosperm- 

 cells remain full. It was further noted that the emptying of the endosrerm 

 was stopped more rapidly by immersing the storage region in a solution of sugar 

 than by employing a small quantity of water. The same method of emptying 

 reserve stores was also employed by Puriewitsch in the case of isolated cotyle- 

 dons, root tubers, rhizomes, bulbs, and branches. [Compare Wachter, 1905.] 

 It might also, perhaps, be possible by appropriate methods to induce an empty- 

 ing of an isolated foliage-leaf filled with products of assimilation. In the case 

 of the storehouses mentioned, another highly important method of investi- 

 gation into the migration of reserves may be successfully carried out, for which 

 endosperm is obviously not suited, owing to the fact that it dies off after the 

 emptying of its cells. As already noted we may prevent translocation by 

 means of a sugar solution of appropriate concentration, but if the concentration 

 be increased one finds the opposite process taking place, for the sugar enters 

 the storage tissue and there forms starch. This phenomenon is identical with that 

 recorded at p. 112, where it was seen that the formation of starch took place in 

 the foliage leaf when sugar was supplied from without. In this experiment there 

 are two points of interest for us which we have not previously paid attention 

 to. In the first place, it shows us that the direction of the nutritive stream 

 is determined by the degree of concentration at two different points. Whether 

 an outflow or an inflow of materials takes place in any cell depends, at least 

 to a certain degree, on its surroundings ; both an outflow and an inflow may take 

 place at the same time if the contents of the neighbouring cells on one side show 

 a higher, on the other a lower, degree of concentration of the nutrient in 

 question. Just as in the case of the single cell, so also a tissue situated 

 between two other tissues with different sugar concentrations will permit 

 sugar to stream through so long as this difference is maintained. In 

 other words, physical conditions well known and easily understood govern 

 the situation in this case. The refilling of empty storage regions is in- 

 structive from a second point of view. It shows that a continuous removal of 

 the sugar formed, using a very large amount of water in the emptying experi- 

 ments, is by no means always necessary for the maintenance of the diffusion 

 current, and that the translocation may be replaced by storage^ and trans- 

 formation. In fact, the movement of materials into an empty cotyledon 

 would soon stop, were it not that the entering sugar is changed into starch, 

 and thus room is made for new supplies. It need scarcely be mentioned 

 that the principle of diffusion already mentioned, i. e. the maintenance of the 

 flow by translocation or storage, is not limited to the case of sugar and starch, 

 where it can be conveniently demonstrated, but that it applies to all other 

 migratory substances. The principle is by no means a new one ; on the 

 contrary, it has been discussed in detail in relation to the osmotic characters 

 of the cell, although its importance may be again emphasized here. 



The diffusion flow is, however, not the only important factor in the migra- 

 tion of materials ; the permeability of the protoplasm is of equal importance. 

 It would be unfortunate for the plant if its cells permitted all the reserves they 

 collected to diffuse outwards in the way they do in Puriewitsch's researches 

 on storehouses of reserve. A rapid streaming of materials would then take 

 place towards the roots, and from them into the soil ; the existence of the plant 

 would then become impossible. If the plant is not to lose all its reserves by 

 diffusion, the external walls, which are in close relation to water, must not 

 permit of the passage of the reserves through them. In all probability this 



