l] WHAT IS PLANT GEOGRAPHY ? I3 



(meristems) situated near the apices of stems and roots. Fig. 19 (B 

 and C) shows stem-sections of higher plants with the main types 

 of tissues and examples of their disposition. 



The above references are chiefly to the more or less solid walls of 

 plant cells. But all these cells are alive, at least in youth — for they 

 contain a viscous and very heterogeneous fluid known as protoplasm, 

 which is the living matter of the plant. It is in the protoplasm that 

 occur the extremely complex sequences of events which integrate 

 into what we know as life, and which include the processes enabling 

 the protoplasm to increase itself. This increase forms the basis of 

 growth, which normally involves increase in size of the cell until 

 it reaches a maximum and thereupon divides into two daughter 

 cells. The daughters then repeat the process, and as a result of 

 numerous repetitions of this activity the plant as a whole grows in 

 size. Another activity going on in all living cells is the slow oxidative 

 ' burning ' known as respiration, which gives to living organisms the 

 energy required for their life-processes. 



Besides the apical meristems by which plant organs grow in 

 length, there is, in the stems and roots of many long-lived higher 

 plants, a layer of actively dividing cells (the ' cambium ') which add 

 daughters radially on either side and so lead to growth in girth. 

 When this takes place year after year in regions of fluctuating 

 climate, where cells of different sizes are produced at diff^erent 

 seasons, annual ' growth-rings ' are formed which may easily be 

 seen in most timbers. In addition there are meristems in buds 

 whose behaviour — varying from dormancy to active elongation — 

 greatly affects the ultimate shape of plants. These and other growth 

 phenomena are largely controlled by special chemical substances 

 produced by the plant, and in ways which are only nowadays being 

 elucidated. These plant growth substances, for example, may 

 stimulate the elongation of cells in some tracts while inhibiting that of 

 others — resulting in curvature of an organ in relation to a directional 

 stimulus, such as light, which itself affects the production or 

 availability of the chemical stimulant. Other substances inhibit 

 growth, an example being produced by many terminal buds ; 

 accordingly it is only when such inhibitors are removed that the 

 lateral buds grow out actively (hence the sprouting of a hedge after 

 clipping, and of pasturage after close grazing). For a general survey 

 of this fascinating and important subject, see Professor L. J. Audus's 

 Plant Growth Substances, second edition (Leonard Hill, London, 

 pp. xxii + 553, 1959). 



B 



