DIV. ii PHYSIOLOGY 243 



by DIFFUSION and by CONDUCTION. Necessary conditions for diffusion 

 are that the cell wall and protoplasm should be permeable for the 

 substance in question, and that there should be a difference in its 

 concentration between the starting place and termination. In the 

 transport from one vacuole to that of the neighbouring cell the 

 substances must first pass into the protoplasm, then into the cell 

 wall, then again into the protoplasm, and finally into the vacuole. 

 The cell walls, at all events when thick, appear to offer special 

 difficulty in the process. On this account all thickened cell walls 

 are provided with thin places (pits), and the pit membranes are 

 traversed by fine protoplasmic threads (plasmodesms, p. 44). In 

 the sieve -tubes the pit membrane is absorbed, and thus coarser 

 strands of protoplasm connect the one cell with its neighbour. The 

 investigations of BROWN and ESCOMBE have shown that a finely per- 

 forated septum, if the perforations are a certain distance apart, offers 

 no obstacle to diffusion ( 25 ). 



Movements of diffusion may also take place within a cell if dissolved 

 substances are not at the same concentration throughout the cell. 

 Movements of diffusion proceed quite slowly. The rapidity with 

 which mixing occurs may be greatly hastened if a movement in 

 mass be added to that due to diffusion. In common life and in the 

 laboratory this is effected by shaking the solution, and within a cell 

 the same result may be obtained, e.g. by the protoplasmic movements. 

 The greater the length of a cell the more suitable is it for conducting 

 material through the plant, since the slow diffusion movement need 

 only take place at long intervals, i.e. at the ends of the cell in the 

 intermediate portion movements of mixing play a large part. 



When a plant requires more rapid transport of materials the 

 nutrient salts are conveyed in the plant by the transpiration current. 

 It is thus not merely water but a very dilute food-solution that is 

 conducted by the vascular bundles, and the use of transpiration is, in 

 the first place, to concentrate this nutrient solution and, in the second, 

 to bring it quickly to the proper parts of the plant. Apart from this 

 result it would be difficult to understand the process of transpiration, 

 and the plant would certainty have found means of limiting it. When 

 it is actually checked (cf. p. 168), we have to do with plants which 

 grow slowly on account of the poor supply of salts, and also it is true 

 of carbonic acid. 



Nutrient Salts and Agriculture. Since the plant thus continues 

 to absorb nutrient salts from the soil, this must become poorer in the 

 particular substances unless the loss is repaired in some way. In 

 nature this results from the fallen and dead parts of plants returning 

 to the soil, and the salts contained in them becoming available for 

 further life. In agricultural practice, however, a large proportion of 

 the vegetation is removed in the crop, and the salts it contains are 

 thus lost to the ground ; at the most a fraction may be returned to 



