43 



MORPHOLOGF OF THE CELL. 



it is denser than the inner and more watery substance. That the cohesion tn each pro- 

 toplasmic body decreases from without inwards, follows from the easier mobility of the 

 inner mass, which is especially the case with the plasmodia, and also from the formation 

 of vacuoli, which clearly depends on the collection of a portion of the water present 

 in the protoplasm round internal points, and the final formation of drops there, pre- 

 supposing -that the cohesion is overcome at these points. The view here presented 

 that the hyaline homogeneous original substance itself is formed on each free surface of 

 motion of the protoplasm as a skin destitute of granules, entirely agrees with the sup- 

 position that not only every vacuole in a solid protoplasmic body, but also every thread 

 of protoplasm which penetrates the sap-cavity, and finally the inner side of the proto- 

 plasm-sac which encloses the sap-cavity, is also bounded by a skin, even if it be so thin 

 that it cannot be seen when strongly magnified \ 



If the protoplasm is not enclosed in a cell- 

 wall, the vacuoli are usually small and not 

 numerous ; if, on the other hand, a cell-wall 

 is formed and if the cell grows rapidly, this is 

 always accompanied by an increase in number 

 and size of the vacuoli (Fig. i). This not 

 unfrequently leads to a frothy condition of 

 the protoplasm where the vacuoli are sepa- 

 rated only by thin lamellae of that substance 

 (Fig. 43, ^) ; but in other cases the inner 

 protoplasmic mass of a cell breaks up into 

 smaller portions, each of which encloses a 

 large vacuole, which is surrounded by a thin 

 membrane of protoplasm (Fig. 43, B, b). 

 These are the 'sap-vesicles' (Saftbliischen) 

 which so commonly occur, and which some- 

 times enclose chlorophyll and other grains, 

 and thus become similar to cells (not un- 

 common in the flesh of berry-like fruits, and 

 in tissues with mucilaginous juices). If the 

 rapidly growing cell does not form new pro- 

 toplasm, /. e. if its protoplasmic body is not 

 correspondingly nourished, then, in propor- 

 tion as the size of the cell and the amount 

 of sap increase, the quantity of protoplasm 

 decreases ; and not unfrequently it forms a thin sac not directly visible, lying between 

 the cell-wall and cell-sap, clothing the former like thin tapestry, and becoming visible 

 only by means of substances that remove the water, and loosen the protoplasm-sac 

 (Primordial Utricle of Mohl) from the cell-wall by contraction (Fig. 43, C, p). The 

 signification of this thin-walled protoplasm-sac, its production by increase in number 

 and size of the vacuoli in an originally solid protoplasmic body, will no longer be 

 doubtful to the reader after all that has been said in sects, i, 2, and 3, and by com- 

 parison of Fig. I with Fig. 43. 



In younger cells, where the protoplasm forms a still thicker layer, or where it 

 presents a net-work permeated by vacuoli, its substance, with the exception perhaps 

 of the outermost layer lying on the cell-wall, appears to be always engaged in a 

 'streaming' movement, which is however usually very slow. In many mature and 

 large cells this condition is permanent, when they do not serve for the storing up 

 of assimilated materials, and when the protoplasmic body is sufliciently nourished. 



Fig. 43.— Forms of the protoplasm contained in cells. A 

 and B of Zea Mais; A cells from the first leaf-sheath of 

 a germinating plant ; B from its first internode ; C from the 

 tuber of Heliaiithiis tuberosus, after action of iodine and 

 dilute sulphuric acid; h cell-wall ; k nucleus ; p protoplasm. 



' Cf, Hanstein, Die Bewegiingserscheinuiig des Zellkerns, u. s. 

 Theinischen Gesellschaft z\\ Bonn, p. 224, 1870. 



Sitzungsberichte der niecler- 



