332 LECTURE XV. 



shall see, the expression of the organising activity may 

 be more or less modified by external circumstances. The 

 various degrees of histological differentiation presented by 

 plants is simply the expression of a greater or smaller similarity 

 in the organising properties of the protoplasm of the cells of 

 which each plant consists. For instance, the greater thick- 

 ness of the wall of one cell as compared with that of another, 

 though they exist side by side and under the same conditions, 

 is due to differences in the organising properties of the pro- 

 toplasm in the two cases. Further, the differences in form 

 exhibited by starch-grains are to be traced to a similar cause : 

 the difference in form between the starch-grain of the Wheat 

 and that of the Potato, for example, can only be accounted 

 for by referring it to a difference in the. organising properties 

 of the amyloplasts in the two plants. 



We may begin our more detailed study of growth by 

 briefly .enumerating the general conditions upon which it 

 depends. With some of these we are already familiar. We 

 know, for instance, that growth can only take place when the 

 growing organ is adequately supplied with material in the 

 form of what we have termed the plastic products of the 

 metabolism of the plant. Growth is then ultimately de- 

 pendent, so far as the necessary material is concerned, upon 

 constructive metabolism. We know also that .growth is 

 associated with active destructive metabolism ; that it is de- 

 pendent, in the case of aerobiotic plants, upon the absorption 

 of free oxygen, and, in the case of anaerobiotic plants, upon 

 a supply of fermentable substance. Growth is then depend- 

 ent, so far as the necessary energy is concerned, upon de- 

 structive metabolism. Again, it was shewn at length in the 

 last lecture that growth can only go on within certain limits 

 of temperature. Finally, there is one other essential con- 

 dition, and that is an adequate supply of water to the growing 

 organ : this is of importance in order that the cell or cells 

 may be in a state of turgidity (p. 40), without which, as we 

 shall see, growth is impossible. 



It must not be supposed, however, that, provided that an 

 organ is supplied with the necessary material, that the neces- 



