34 THE ELEMENTARY STRUCTURE OF PLANTS. 



cells which compose the jointed hairs of the common Spider Lily 

 (Tradescantia, Fig. 6), show this circulation well, under a magni- 

 fying power of about four hundred diameters. With this power, a 

 network of anastomosing currents, rendered visible by the little 

 globules they carry with them, will be seen to move between the 

 transparent and glassy cell-membrane and the inclosed colored 

 contents, traversing the cell in various directions, without much 

 regularity, except that the streamlets appear to radiate from, and 

 return to, the parietal cytoblast (28). In this instance, it is easy 

 to see that the currents belong to the layer of mucilaginous fluid, 

 or protoplasm, interposed between the cell-membrane and the 

 colored aqueous contents. The same is the case, according to 

 Mohl's thorough observations, in the tubular cells of Chara, where 

 they may be observed with an ordinary lens ; and in our Vallisne- 

 ria, where a moderate magnifying power shows, in the cells of the 

 leaves, a continuous rotation round the whole wall of the cell, the 

 stream rising on one side and descending on the other. The cur- 

 rent is powerful enough to carry along, not only minute granules, 

 but small grains of chlorophyll or green coloring matter (87), 

 which renders it abundantly visible ; and sometimes, where the 

 green granular contents cohere in a mass filling the centre of the 

 cell, it throws this whole mass into slow revolution on its axis. In 

 these instances, the whole layer of mucilaginous fluid takes part in 

 the movement. The cause of this motion is wholly unknown, as 

 also the office it subserves. We shall have occasion to refer to it 

 in another chapter, in connection with other vegetable movements. 

 At present, we may merely remark that it is not like a true circu- 

 lation, through vessels, which is characteristic of animals. 



37. Permeability and Imbibition, The wall of the cells, at least in 

 -their living or vitally active state, is a perfectly closed sac, desti- 

 tute of openings or visible pores (although perforations sometimes 

 appear in old or effete cells, as in those of Peat-Moss) ; but, like 

 all organic membranes, it is permeable to fluids. The cell con- 

 stantly contains a fluid thicker than water, and therefore tends to 

 imbibe water by endosmosis,* as well as to yield by exosmosis * a 



* Endosmosis and exosmosis are names given by Dutrochet (who first illus- 

 trated them in liquids^ to a physical process of permeation and interchange 

 which takes place in fluids, according to the following law, briefly stated. 

 When two liquids of unequal density are separated by a permeable mem- 

 brane, the lighter liquid or the weaker solution will flow into the denser or 



