484 PLANT GROWTH AND PLANT COMMUNITIES 



influence upon the absorbed and redistributed solute, whereas the 

 growing leaf in the dark may not do so ( cf. work of Millar and Pollock 

 in Figure 64B of Steward and Sutcliffe). Without this pull at the 

 "sink," no active movement along the channels of transport would be 

 of much avail. We are, however, quite at a loss to know what form of 

 signal passes from the accumulating cells at the "sink" to the cells at 

 the source to prompt them to release their previously accumulated 

 solutes. 



Two further thoughts are relevant. The role attributed to the cam- 

 bial region in dicotyledons, which draws off solutes laterally and 

 furnishes them to growing regions above, can be played in monocotyle- 

 dons by the intercalary meristems in the ensheathing bases of leaves 

 (Steward, 1954). Secondly, the workers on translocation may have 

 been too preoccupied with the characteristics of mature elements of 

 xylem or phloem, regarded as the principal channels of transport over 

 long distances. On the philosophy here outlined, the newly formed 

 cells of the cambial and procambial region— without sharp distinction 

 between xylem and phloem mother cells— have essential prerequisites 

 for a role in rapid transport, and the mature sieve tube would then 

 suggest the "remnant, like a dried-up river bed, suggestive of the place 

 where flow once occurred." 



Thus the plea is made here for a return in these problems of plant 

 physiology to their reconsideration in terms of growth and develop- 

 ment as the all-important background against which the cellular physi- 

 ology and biochemistry will become more meaningful. 



Cell physiology and morphogenesis 



Morphogenesis is the response of the growing system to certain 

 controls, external and internal, and it is natural to see those controls 

 as mediated by chemical growth regulating substances which make 

 their effects apparent upon growth either by cell division or by cell 

 enlargement. This again suggests areas of reponse in which it is im- 

 portant to understand the cells as organized systems capable, by virtue 

 of this organization, of manifold interlocking functions, such as protein 

 synthesis, salt and water intake, etc. 



Stimuli to growth. The most important stimulus to growth and 

 morphogenesis is that which follows upon fertilization of the egg. The 

 zygote grows apace, but in angiosperms the product of the other nu- 

 clear fusion, which gives rise to endosperm, may often outpace the 

 young embryo. Thus there are laid down, precociously and in advance 

 of the embryo's needs, the nutrients and sources of stimuli that will 

 nourish the zygote. In the present context the point of this observation 

 is that these materials, especially the liquid content of the endosperm, 



