472 MANUA.L OF BOTANY 



parenchyma of leaves, the stellate cells of the pith of certain 

 rushes, the laticiferous cells of the Spurges, &c. 



Growth may, in the light of the considerations advanced 

 above, be defined as permanent increase of bulk attended by 

 permanent change of form. We must*not assume that increase 

 of bulk is necessarily growth, for, as we shall see, in growing 

 cells and members there is a constant stretching of the cell or 

 tissue by hydrostatic pressure or turgidity which can be dis- 

 tinguished from growth by the fact that it can be removed, 

 with the result of a certain amount of shrinkage of size of the 

 part under examination. 



Growth in the lowliest plants may be coextensive with the 

 plant body. In all plants of any considerable size, however, it 

 is localised in particular regions, and in them it is associated 

 with the formation of new protoplasts. We have already seen 

 in the case of the sporophytes of all the higher plants that there 

 exist certain regions in which the cells are merismatic, that is, 

 which have the power of cell-multiplication by means of 

 division. In such regions, when a cell has reached a certain 

 size, which varies with the individual, it divides into two, each 

 of which increases to the original dimensions and then divides 

 again. These regions have been called growing points ; they 

 may be apical, or intercalary, or may consist of definite layers 

 known as cambium layers, or phellogens. By the activity of 

 the protoplasts in these merismatic areas the substance of the 

 plant is increased. As they consist of cells, it is evident that 

 the growth of the entire organ or plant will, depend on the 

 behaviour of the cells of which it is composed. 



The growth of a cell will be found to depend mainly upon 

 four conditions : 1. There must be a supply of nutritive or 

 plastic material at the expense of which the formation of proto- 

 plasm can take place. 2. There must be a supply of water to 

 such an extent as to set up a certain hydrostatic pressure in the 

 cell. This condition we have already considered in the iirst 

 chapter of this section, where we discussed the relation of proto- 

 plasm to water. In the absence of this ticrgescence no growth 

 is possible, for reasons that will presently appear. ■ 3. There 

 must be a certain temperature in the plant, for the activity of 

 protoplasm can go on only within certain limits, which differ in 

 the cases of different plants. 4. There must be a supply of 

 oxygen to the growing cell, for, as we have seen, the protoplasm 

 is dependent upon this gas for the performance of its vital 

 functions. This is evident from the consideration that the 



