

SECT. II PHYSIOLOGY 187 



materials is still more complicated in a higher plant composed of 

 many cells, the root of which is embedded in the damp soil, while the 

 leafy shoot is expanded in the air. Not only does the possibility of 

 exchange of substances differ essentially in the subterranean parts 

 from that in the portions above ground, but there are all the internal 

 cells to be considered that do not communicate directly with the 

 environment ; these have special conditions governing the exchange of 

 substances between them. Extensive movements of material must 

 necessarily take place within such a plant. The principles underlying 

 these movements, the causes of the translocation, must first be con- 

 sidered generally. 



Translocation of substances may either be from one cell to another, 

 or it may go on within a limited space in the plant, such as a single 

 cell or tube, e.g. in vessels or in intercellular spaces. The movement 

 from cell to cell can only take place by diffusion, and the 

 transported substance must either be water, or a solid or gaseous 

 substance dissolved in water. The passage from one cell to another 

 takes place in exactly the same manner as did diffusion into the cell 

 from the outside. The existence of a difference in concentration 

 between the two cells is a necessary condition ; this may be due 

 either to the protoplasm or to the cell-sap. If, for instance, the 

 same substance is present in different degrees of concentration in the 

 sap of two cells, and if the protoplasm is impermeable to the particular 

 substance, water will, as a rule, diffuse from the more concentrated to 

 the less concentrated cell -sap until the same concentration is 

 attained in the two. We may say that the one cell attracts water 

 osmotically from the other. If, however, the protoplasm is permeable 

 to the particular solution, a diffusion of the salt will also take place 

 from the more concentrated to the less concentrated solution. With 

 permeability of the protoplasm a diffusion from cell to cell is also 

 possible when the concentration is the same, but the substances are 

 different ; a condition of equilibrium will then be reached Avhen both 

 substances are equally distributed between the cells. If, however, 

 one of the substances cannot pass through the protoplasm, it will 

 remain in the cell in which it was, and only the other substance will 

 be distributed equally between the two cells. 



In the transport from one vacuole to that of the neighbouring cell, 

 which has just been considered, the substances mi;st first pass into 

 the protoplasm, then into the coll-Avall, 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-Avalls are provided with thin places (pits), 

 and the pit-membranes are traversed by fine protoplasmic threads 

 (plasmodesms, p. 97). In the sieve-tubes the pit-membrane is 

 absorbed, and thus coarser strands of protoplasm connect the one cell 

 with its neighbour (p. 98). Tje investigations of Brown and Escombe 



