216 VEGETABLE PHYSIOLOGY 



its manufacture. The cell or protoplast, which contains a 

 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, groivth is proceed- 

 ing. This may result in a continuous increase in the dimen- 

 sions 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 variable 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, 

 though 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 different 

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

 toplast 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 intermittence 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 re- 

 moved and growth should again proceed, there will be no 

 stream of constructive material, and much time will be lost 



