210 VEGETABLE PHYSIOLOGY 



portion of the chlorophyll apparatus, has thus not only to 

 provide for its own nutrition, but to prepare a part of the 

 nutritive material required by other protoplasts which are 

 set apart for the discharge of other work. 



But this is not all. We find, from a study of plants, 

 that in almost all cases, so long as life lasts, growth is 

 proceeding. This may result in a continuous increase in 

 the dimensions of the plant-body, or may lead only to the 

 replacement of parts which have a brief existence, and 

 need to be renewed. This is the case, for instance, in 

 forest trees that have attained their full dimensions. 

 Growth in the vegetable organism is very definitely 

 localised. Growth in length takes place at or near the 

 apices of stems and roots ; it has a definite though vari- 

 able localisation in leaves of different kinds. Growth in 

 thickness is confined to sheaths or bands of cells in different 

 regions of the axis, such as the cambium, and the different 

 phellogens met with in the cortex. 



Growth and nutrition differ in another respect : the 

 former is intermittent, the latter needs to be constant, 

 chough the intensity of the requirements may vary. 



These considerations show us that there must exist in 

 the plant a very complete mechanism by which the differ- 

 ent food-stuffs can be circulated about its body. Each 

 protoplast must be in receipt of a continuous, though per- 

 haps small, supply of nutritive material ; the demands 

 of growth must be satisfied by the transport of considerable 

 quantities of formative material to the growing regions. 

 The inter mittence of growth makes a further demand. 

 Consider one among many places at which a large con- 

 sumption of such formative material is proceeding : a stream 

 is travelling there to supply the need. Suppose that some 

 temporary check to the growth at that spot takes place. 

 The stream will be diverted elsewhere by the demands of 

 the other growing parts, and when the hindrance is 

 removed and growth should again proceed, there will be no 

 stream of constructive material, and much time will be lost 



