726 PHYSIOLOGY. 



may be readily observed when they are submitted to a moderate 

 microscopic power; but here, the protoplasm with its granular 

 contents, will be seen to pass round the interior of the walls of each 

 cell {fig. 1100). In the Characece again, and especially in the 

 Nitellce the more transparent forms of such plants, the moving 

 protoplasm does not rotate round the walls, or in reticular 

 currents, but it passes obliquely up one side of the cell (Jig. 1094) 

 until it reaches the extremity, and then flows down in an 

 opposite direction on the other side. 



No satisfactory explanation has as yet been brought forward 

 to account for this movement of the cell-contents, all that we 

 know is, that it is connected with the vitality of the plant, and 

 hence all agents that actually injure the cell will stop it at once 

 and permanently. The movements of the ciliated zoospores of 

 the Algae (see p. 389 soi&figs. 829 and 830), and those of the 

 ciliated spermatozoids of the Algse (see p. 393 axi^fig. 838) and 

 of the higher Cryptogamic plants (see p. 370 and^^f. 785), are 

 regarded by Henfrey and others, as " analogous to the I'Otation of 

 the protoplasm." 



4. Elaboration of the Cell-contents. — All cells exposed to light 

 and air which contain a primordial utricle, have the power of 

 producing in their contents, the various azotized and unazotized 

 compounds which are concerned in the development of new 

 tissues, and in the formation of the various secretions of the plant. 

 (See Bespiration and Assimilation.) In old cells the secretions 

 of the plant are also, in part, deposited. 



2. Ftjnctigns of Prosenchyma. — Prosenchymatous cells are 

 especially adapted by their construction and mode of combi- 

 nation into a tissue, for giving strength and support to plants; 

 hence there can be no doubt, but that this is one of the oflSces 

 which they perform. In a young state, also, before such cells are 

 filled with secondary deposits, they appear to be the main agents, 

 by which the fluids absorbed by the roots, are carried upwards 

 to the leaves and other external organs, to be elaborated by the 

 agency of light and air. The experiments of Hoffman, Unger, 

 and others, would seem clearly to prove this. Thus, IIoflFman, 

 by placing plants in such a situation as to cause them to absorb 

 a solution of fcrrocyanide of potassium, and then adding a per- 

 salt of iron to sections of them, found that the prussian blue 

 which was formed by the reaction of the chemical agents thus 

 applied, was principally deposited in the prosenchymatous cells. 

 Unger also, came to the same conclusion, by causing plants to 

 absorb a coloured juice, and tracing its passage. It should be 

 noticed, however, that other experimenters, such as Link and Ro- 

 minger, have arrived at diametrically opposite conclusions. 

 (See Vessels, below.) 



3. Fdnctions of Vessels. — The functions of the spiral, annular, 

 reticulated, pitted, and scalariform vessels have been a subject 



