218 VEGETABLE PHYSIOLOGY 



development of succulent parenchyma. During the second 

 year they have a much smaller foliar development, but each 

 sends up its flowering stem. The constructive activity is 

 much less than during the previous year ; the root gradu- 

 ally dwindles as the fruit and seeds develop, the store 

 deposited in the succulent parenchyma being applied to 

 their formation and maturity. 



Based upon considerations such as these, we may make 

 a further classification of the nutritive substances which 

 exist in the body of the plant. We can speak of those 

 which are used in the cells where they are formed, and of 

 those which are removed therefrom for the feeding of the 

 other protoplasts. These, again, may be devoted to imme- 

 diate use, or may be stored as reserve materials for deferred 

 consumption. We can recognise in every plant some kinds 

 which are suitable for transport from cell to cell, and others 

 which are not able to pass through cell-walls, but must 

 remain in the position in which they are formed. These 

 two classes of circulating and stored food-stuffs have an 

 intimate relationship to each other, and must be mutually 

 interdependent, each being reinforced by the other accord- 

 ing to the needs of the particular moment. 



If now we turn from these general considerations to 

 the sequence of events that are normally taking place in 

 the cells which contain the chloroplasts, we can form some 

 definite idea of the course of the processes of construction 

 of the carbohydrates and removal of the products. In 

 such a cell there is, during favourable conditions, a manu- 

 facture of sugar which is continuous and rapid. The 

 cell itself needs a certain amount of such sugar for its 

 own nutrition, but only a very small part of what is being 

 made. The sap of its vacuole soon contains a large excess 

 of sugar, and if nothing further transpires the process of 

 manufacture must stop. But the cell is in contact with 

 others, in many of which, perhaps in all, a similar manu- 

 facture is taking place. The ordinary processes of diffusion 

 or secretion tend to equalise the amounts in any contiguous 



