TISSUES AND SIMPLE ORGANS. 101 



downward along the cortex; wherever the path is broken, this 

 substance is at once converted into callous tissue. 



The movement of undissolved albuminous substances in the 

 sieve-tubes, like that of the milk-sap mentioned above, is a mass- 

 movement. The causes for this movement, though not definitely 

 determined, have already been referred to. Gravity, outer me- 

 clianical pressure upon the soft elements, and turgor- oscillations 

 in the neighboring tissues no doubt assist in bringing about this 

 movement. 



(i. Conduction of Carbohydrates. 



Here, and in general with substances in solution which must 

 pass through plasmic membranes and cell-membranes, we are con- 

 cerned with molecular Tnovements., which belong to the domain of 

 osmosis. General statements only will be made now ; particulars 

 will be given below. 



The physical considerations of osmotic action differentiate (1) 

 hydro-diffusion.^ the osmotic interchange of two miscible substances 

 without any separating membrane ; from (2) diosmosis in the usual 

 sense, tliat is, a process similar to (1) with a dyalizing membrane 

 or porous substance. Both processes occur in the vegetable cell. 



Ad 1. In an assimilating palisade- cell exposed to the sunlight 

 several currents must be formed in obedience to the principle that 

 the current is fonned at right angles to the lines of 

 equal concentration (see Fig. 60). It is assumed 

 that the maximum concentration of sugar, for ex- 

 ample, is at 5 (Fig. 60). The sugar-molecules will 

 then move in the direction of the arrows ; the water- 

 molecules in the opposite direction. Wherever a 

 crystal or a starch-grain grows within a cell, there 

 are produced such zones of concentration in the 

 surrounding liquid, and the respective movements 

 mil take place. 



Ad 2. If the solutions in neighboring cells are of Haberiandt.) 

 unequal concentration, a new complication arises, which leads us 

 into a branch of physiology, in part, yet unexplained. The more 

 recent investigators (Brucke, Pfeffer) have, however, given many 

 explanations and suggestions. Whatever applies to processes that 

 may be traced to living protoplasm is also applicable here ; the 



Fig. 60. 

 (Modified from 



