12 



BOTANY 



PART 



In many animal cells an additional constituent of the protoplast has been 

 demonstrated as a small body which is called a CENTRIOLE, in the immediate 

 neighbourhood of the nucleus. Similar bodies are 

 found in the vegetable kingdom in the cells of some 

 Cryptogams, but are not of general occurrence even 

 in them (Fig. 21 A). 



It is only the embryonic cells of the plant, 

 as they are met with in the apices of stem and 

 root, which are thus completely filled with 

 protoplasm. This does not hold for the fully 

 developed cells of the plant which arise from 

 these by growth in size and alterations of 

 shape. During this transformation to cells 

 of the permanent tissues the embryonic cells 

 of plants, unlike those of animals, become 

 poorer in protoplasm, since this does not in- 

 crease in proportion to the growth of the cell. 

 In every longitudinal section of the growing 

 point of the stem it can be seen that at some 

 distance from the tip the enlarged cells have 

 already begun to show cavities or VACUOLES 

 (v in A, Fig. 3) in their cytoplasm. These 

 are filled with a watery fluid, the CELL SAP. 

 ~ c y The cells continue to increase in size, and 

 usually soon attain a condition in which the 

 whole central portion is filled by a single 

 large sap cavity (v in JB, Fig. 3). The cytoplasm 

 then forms only a thin layer lining the cell 

 wall, while the nucleus occupies a parietal posi- 

 tion in the peripheral cytoplasmic layer (Fig. 

 3 B, k). At other times, however, the sap 

 cavity of a fully -developed cell may be 

 traversed by bands and threads of cytoplasm ; 

 and in that case the nucleus is suspended 

 r" ~| in the centre of the cell (Figs. 5, 10). But 



whatever position the nucleus may occupy, it 

 JT3LJ n th1 Always embedded in cytoplasm ; and there 

 growing point of a phanero- is always a continuous peripheral layer ot 

 gamic shoot, k, Nucleus ; cy, cytoplasm lining the cell wall. This cyto- 



cytoplasm ; v, vacuoles, re- ^ -111 vu 



presented in B by the sap plasmic peripheral layer is in contact with 

 cavity. (Somewhat diagram- the cell wall at all points, and, so long as 

 50<X After the cel1 remains living, it continues in that 

 condition. In old cells, however, it frequently 

 becomes so thin as to escape direct observation (Fig. 10), and is 

 not perceptible until some reagent which attracts water and causes it 

 to recede from the wall has been employed. 



