42 



MORPHOLOGy OF THE CELL. 



for as much as half-an-hour or an hour (Fig. 40). This interpretation of the skin 

 is not at all opposed to the fact that it is denser than the inner and more watery 

 substance. That the cohesion in each protoplasm-mass decreases from without inwards 

 is shown by the greater mobility of the inner portion, especially with plasmodia, and 

 also by the formation of vacuoles, which clearly depends on the collection of part of 

 the water present in the protoplasm round internal points in drops, presupposing 

 that the cohesion is overcome at these points. The view that the hyaline homogeneous 

 basis itself forms on each exposed surface of the protoplasm the skin destitute of 

 granules, entirely agrees with the supposition that not only every vacuole in a pro- 

 toplasm-mass, but also every thread of protoplasm which traverses the sap-cavity, and 

 finally the sap-cavity itself, 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 vacuoles 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 vacuoles 

 (Fig. T, p. 2). This not unfrequently leads 

 to a frothy condition of the protoplasm where 

 the vacuoles are separated only by thin lamellae 

 of that substance (Fig. 41, ^); but in other 

 cases the inner protoplasm-mass of a cell 

 breaks up into smaller portions, each of which 

 encloses a large vacuole surrounded by a thin 

 skin of protoplasm (Fig. 41, 5, b). These 

 are the ' Sap-'vesicles ' which are so com- 

 mon, and which sometimes enclose granules 

 of chlorophyll and other substances, and thus 

 resemble cells ; they are not uncommon in the 

 flesh of berry-like fruits, and in tissues with 

 mucilaginous juices. If the rapidly growing 

 cell does not form new protoplasm 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 im- 

 mediately visible, lying between the cell-wall 

 which it lines and the cell-sap which it in- 

 vests, and becoming visible only by means of 

 reagents that remove the water and separate 

 the protoplasm -sac — the Primordial Utricle of 

 MohP — from the cell wall by contraction (Fig. 41, C,p). The significance of this thin 

 protoplasm- sac, and its production owing to the increase in number and size of the 

 vacuoles in an originally continuous protoplasm-mass, will no longer be doubtful to 

 the reader after all that has been said in Sects, i, 2, and 3, and by comparison of 

 Fig. I with fig 41. 



In young cells, where the protoplasm still forms a thick layer or a net- work per- 

 meated by vacuoles, 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, which do not serve 

 for the storing up of assimilated materials, and where the protoplasm-mass is sufficiently 

 nourished, and does not, as the cell increases in size, contract to a mere thin skin, 



Fig. 41. - Kornis of tlie protoplasm contained in cells 

 ^1 and B of maize ; ^ cells from the first leaf-sheath of 

 a jjenninating plant ; B from its first internode ; Cfrom the 

 tuber of the Jerusalem artichoke, after action of iodine 

 and dilute sulphuric acid ; /t cell-wall ; A nucleus ; / pro- 

 toplasm (primordial utricle). 



^ See Hanstein, Die Bewegungserscheinung des Zellkerns, u. s. w. in Sitzungsberichte der nieder- 

 rheinischen Gesellschaft zu Bonn, Dec, 19, 1870, p. 224. 



[H. von Mohl, Bot. Zcitg. 1844, p. 273.] 



