THE CELL-WALL. CENTRIFUGAL THICKENING 43 



certain Algae which have the power of actively changing their shape. 

 The elaioplasts discovered by Wakker are possibly also vacuolar in 

 character; these structures, which are particularly prevalent in the 

 epidermal cells of OltCHIDACEAE and LlLIACEAE, appear to consist of a 

 protoplasmic stroma containing numerous drops of oil in suspension. 



- The pulsating or contractile vacuoles, finally, which occur in certain 

 Protococcales, and also in the zoospores of all the Myxomycetes, of 

 many Algae and of a few Fungi, seem to be organs mi generis. The 

 mechanism of their pulsation, that is, of their rhythmic disappearance 

 and reappearance, is not properly understood ; it is certain, however, 

 that these structures perform very special functions, which are con- 

 nected in some cases with metabolism, but in others with the process 

 of locomotion. 



6. The Cell-Wall. 



Plants exist in the form of naked protoplasts only during the 

 earliest stages of their ontogenetic development. Thus the zoospores of 

 Algae and Fungi, ascospores and all gametes, are at first devoid of a 

 cell-wall. Again, the synergidae and antipodal cells of the Angio- 

 spermic embryosac, and the pro-embryo-cells produced (by free cell- 

 formation) from the oospore in many Gymnosperms are naked for a 

 time, or even throughout their existence. Apart from such exceptional 

 cases, however, it appears to be a general rule among the Higher 

 Plants that the fertilised egg-cell and all the succeeding generations of 

 cells are from the very first provided with a cell -membrane. 



In the growing-points the cell-walls are always thin and delicate. 

 The conversion of embryonic cells into permanent tissue is almost 

 invariably attended by a more or less pronounced growth in thickness 

 of the cell-walls, usually in relation to increased mechanical require- 

 ments. Comparatively rarely the thickening serves for a different 

 purpose, such as the storage of water in which case it is accompanied 

 by mucilaginous modification or the deposition of nutritive reserve- 

 cellulose, as in the endosperm of the Date and in certain other storage 

 organs. The cell-wall probably never undergoes uniform growth in 

 thickness over the whole of its surface ; in typical cases a number of 

 more or less well defined areas remain entirely unthickened, while the 

 intervening portions grow in thickness either in a centrifugal or in a 

 centripetal direction. 



Centrifugal growth in thickness is necessarily confined to such cells 

 as expose a free surface all round, or at least on one side. The best 

 examples of this mode of thickening are to be found among pollen- 

 grains and spores. The warts, spines, or flanges, often elaborately 

 sculptured or woven into complicated patterns, with which these 



