EXPEESSION OF NUTRITION. 23 



chemical material, thus evolving force which may 

 be observed in action, either at the point where it 

 is produced or at a distance from it. 



Nutrition being a vital, and inscrutable process, 

 we probably cannot know it in the abstract, but 

 we can observe the effects of the process of nu- 

 trition in an organism. The best evidence of the 

 occurrence of nutrition is found in its results. 

 In many cases nutrition is greatly aided by (if not 

 entirely dependent upon) the action of certain 

 external forces afferent to the subject, so that the 

 results in some degree indicate the action of such 

 external forces. We will consider a few examples. 

 In vegetable growth we may find a simple multi- 

 plication of cells, all similar in their histological or 

 structural characters, as is seen in the tissue termed 

 " primary meristem." Here the expression of nutri- 

 tion, and vital action, is multiplication of cells. 

 In a mass of unicellular plants the result, or ex- 

 pression of, nutrition is multiplication of plants, 

 as in the growth of the snow-plant. In other cases, 

 as in the apex of the stem of a plant, when a bud 

 is being formed, differentiation of the cell-growth 

 results from nutrition, producing embryonic leaves. 

 A more complicated case of nutrition expressed by 

 histological changes, is seen in the cellular body 

 called an ovule, which is found in the carpel of 

 a flower. When the protoplasmic contents of a 

 pollen grain have entered that cellular body, changes 

 follow which cause an embryo to develop by cell- 

 differentiation in the ovule ; the structural changes 

 observed in the development of the embryo are the 



