LECTURE IX. 



THE METABOLISM OF PLANTS. 



IN the first lecture we found that when the Yeast-plant is 

 adequately supplied with food-materials, it is capable of 

 forming from them proteid and cellulose. Inasmuch as the 

 size of the Yeast-plant is limited, the result of this formation 

 of proteid and of cellulose is the production of new cells, 

 each of which consists, like the parent, of a cell-wall and 

 protoplasmic contents : and further, inasmuch as the Yeast- 

 plant is unicellular, each of the newly formed cells is a distinct 

 individual plant. In the case of multicellular plants, the 

 result of constructive metabolism is, as in the Yeast-plant, 

 the formation of new cells, but here the new cells remain in 

 connexion with and form part of the plant, and tend to 

 increase its size and weight. For example, if a seedling 

 be placed under such conditions that it can take up and 

 assimilate food, it will be found that it grows and that its 

 dry weight gradually increases to many times that of the 

 seed from which it was developed, the excess of weight being 

 a measure of its constructive metabolism. 



But the whole of the organic substance formed by the 

 plant is not used for the building-up of its tissue. A certain 

 portion of it undergoes decomposition in the processes which 

 we include under the head of destructive metabolism, and 

 certain of the products of decomposition are excreted by 

 the plant. So the increase in weight is not an absolute 



