8o 



LECTURE VI. 



we may assume as probable that they are very finely divided nutritive matters which 

 are employed in the vital processes. 



In the tissue of growing points and embryos, the protoplasm usually fills 

 up the entire space enclosed by the thin cell-walls, and shares it as yet only 

 with the relatively large nucleus (compare Fig. 72). As Fig. 75 shows, how- 

 ever, the protoplasm assumes different forms with the growth of the cells. 

 As the cells become larger, the protoplasm does not increase in an equal 

 degree, but there are formed in it cavities, filled with fluid, i.e. with cell- 

 sap. The protoplasm itself then assumes the form of plates and threads, united 



in a net-like manner, which 

 radiate from a central clump 

 investing the nucleus, to the 

 periphery of the cell, there 

 to pass over into a more 

 or less thin layer of proto- 

 plasm, which here clothes the 

 cell-wall like a hollow sac, 

 The more the cells enlarge 

 with progressive growth, the 

 larger the sap-vacuoles in the 

 protoplasm become ; until, very 

 often, there remains of the 

 latter only a thin lamella 

 lying on the cell-wall, some- 

 what as a wall-paper on a 

 chamber wall. The elonga- 

 tion of the cells, as we see, 

 is essentially connected with 

 the absorption of water, with- 

 out a corresponding increase 

 of the protoplasm. In very 

 succulent parts of plants it 

 is therefore by no means easy 

 to see the protoplasm with 

 the microscope: in the large 

 parenchyma cells of the root-cortex, the pith and the cortex of vegetating shoots, 

 in leaves, fruits, and so forth, it often requires special methods of investiga- 

 tion to bring into view the exceedingly thin protoplasmic utricle which encloses 

 the large sap-cavity of the cell, and lies close upon the cell-wall. This is 

 accomplished by means of various contracting media, e.g. dilute solutions of 

 iodine, alcohol, glycerine, &c., by which the protoplasm is killed, and compelled 

 to separate from the cell-wall in the form of a pellicle, sometimes exceedingly 

 thin. 



It often happens, however, that, with vigorous growth of the cells, and therefore 

 with considerable enlargement of their cavities, the protoplasm also becomes 

 highly nourished and increased in mass. In such cases there are found, even in 



FIG. 77. — Stellate hair on the calyx of the young flower bud of Althaa rosea. 



