THE VITAL PROPERTIES OF THE CELL 71 



jeet, but which may sometimes be demonstrated by means of 

 reagents. Hence, since this layer moves in a certain direction 

 over the siliceous shell, or cellulose membrane, the small organisms 

 can " move in a gliding or creeping fashion over a solid surface " 

 (Engelmann). 



b. The movements of Protoplasm inside the Cell Mem- 

 brane. This kind of movement is chiefly seen in the vegetable 

 kingdom, and as a rule is best observed in the cells of herbaceous 

 plants rather than in those of shrubs and trees. According to 

 de Vries (III. 25), these movements are never totally absent in 

 any plant-cell, but frequently they are so slow as to escape direct 

 observation. They are best seen in vascular tissues, and in those 

 where materials have been stored up, and further at such times 

 when considerable quantities of plastic substances are being 

 transported in order to supply the material necessary for the 

 continuation of growth, for local accumulations, and for special 

 needs (de Vries). Hence this movement of the protoplasm ap- 

 pears to be directly of importance during the conveyance of 

 materials from one part of the plant to another. More rarely 

 it may be seen in the lower organisms, and in the animal king- 

 dom, as in Noctiluca in the vesicular cells in the centre of the 

 tentacles of Coelenterata, etc. 



Two kinds of movements may be distinguished in plants, 

 Hotation and Circulation. 



These movements of rotation were first observed in 1774 by 

 Bonaventura Corti (I. 8); after that they were lost sight of for a 

 time, but were re-discovered by Treviranus. The most suitable 

 objects for studying them are afforded us by the Characeae ; root- 

 hairs of the Hydrocharis morsus ranee, and of Trianea bogotensis, 

 leaves of Vallisneria spiralis, etc., are also very convenient for 

 observations. In the large cells of the Characece, the protoplasm, 

 as has already been described on p. 33, is spread out as a thick 

 cohesive layer upon the inner surface of the cellulose membrane, 

 surrounding the large quantity of cell-sap like a closed sac. In 

 this lining two distinct layers of protoplasm can always be dis- 

 tinguished : an outer one, touching the cellulose membrane, and 

 an inner one, in contact with the cell-sap. The former is always 

 motionless ; in Hydrocharis it is very thin, in Characece it is 

 somewhat thicker, and it also contains a greater number of 

 chlorophyll grains, which remain motionless. This immotile 

 layer gradually passes over into the inner motile one, which in 



