THE THEOEY OF WATEEING 209 



in the formation of new organic substance, but that it serves 

 also to transport the mineral salts contained in the soil to the 

 plant. These are used up by the plants more rapidly at one 

 time than at another, and the plant will, therefore, sometimes 

 take up more of the solution, sometipies less. 



From experiments made on the transpiration of plants, we 

 know that every plant draws most largely on the soil during 

 the putting forth of new shoots and leaves, and takes up most 

 mineral substances at this time. At this period, even plants 

 which grow in arid regions like and require considerable sup- 

 iplies of water. As soon, however, as the shoot is developed and 

 the formation of flower-buds is expected, the watering may be 

 decreased. 



The formation of flowep-buds is best initiated by preserv- 

 ing a period of rest, and the latter is favoured by a diminished 

 water supply. 



This is the reason why cactus-growers let their plants almost 

 shrivel up after the shoots have been produced, and for the same 

 reason pine-apples receive less water, when the formation of 

 fruit is desired. If water is freely given, and no resting 

 period is allowed, leaf development alone takes place, as the 

 tip of the shoot grows on continuously, and remains the centime 

 of attraction for the ascending sap. 



This condition is not desired by the grower, but does not 

 injure the plant. In other cases, however, untimely watering 

 may produce actual disease. This may occur, for instance, 

 when the period of rest at the end of the vegetative season has 

 been disregarded. 



The activity of a plant during one vegetative season may 

 be divided into two periods, and this is the case whether the 

 vegetative season is our summer, or, in the case of tropical 

 plants, falls in with our autumn and winter. With the awak- 

 ening activity, the first purely vegetative period commences, 

 during which the shoot is produced, and begins during the 

 first few months to accumulate food material. In the case of 

 annuals, the organic material which has been formed in the 

 leaves is immediately used up during the flowering period, 

 which follows upon the period of leaf expansion. The food 

 material is immediately and continuously withdrawn from the 



