LECTURE XV. 



X( 



GROWTH. 



WE have already learned, from our brief consideration of 

 the subject in the last lecture, something about growth. We 

 understand at least what we mean by growth. We use this 

 word to express the permanent (i.e. irreversible) changes of 

 form, usually accompanied by a permanent increase in bulk, 

 which are established in the course of the development of 

 plants and their organs. Each plant and each part of a plant 

 has an inherent tendency to assume a certain ultimate form, 

 but this ultimate form is only gradually assumed. All plants 

 and all parts of plants begin their existence with a form 

 which is different from that which they possess when mature, 

 and the higher the morphological differentiation of a plant 

 the greater is this difference, and the more numerous the 

 stages which intervene between the primitive and the mature 

 form. 



I have been careful to say that change of form is usually 

 connected with increase in bulk, for this connexion is by no 

 means necessary. It is easy to imagine that change of form 

 may be effected, not by the addition of material, but simply 

 by the redistribution of material. 



The general statements made above with regard to plants 

 and their organs are true also of the individual cells of multi- 

 cellular plants, and not only of cells but of all organised 

 structures, cell-walls and starch-grains for instance. The 

 form which a cell shall ultimately assume depends upon 

 the organising properties of its protoplasm, though, as we 



