PLANT STRUCTURE 145 



in large amounts are often detrimental to plants. Light 

 applications given frequently are better for healthy plant 

 growth. When plants are given more food than they can 

 readily assimilate, a diminished growth results and if they 

 live, the plants are susceptible to disease. 



Often when plants are highly fed, certain elements 

 are not absorbed by the root-hairs. They remain in the 

 soil, increasing the density of the soil water, and thus pre- 

 vent a healthy growth. This quite frequently occurs in 

 rose soils where the plants are heavily fed with nitrogenous 

 fertilizers. It is especially essential that the rose beds 

 or benches have excellent drainage so that this excess 

 of salts, which becomes detrimental to healthy growth, be 

 washed out of the soil by the application of clear water. 



Definite rules for the application of fertilizers are diffi- 

 cult to give because plants of different species have such 

 varying requirements. 



111. Atmospheric conditions. The second source of 

 plant food is from the air and is a most important one. 

 The living protoplasts within the protoplasm make use of 

 the carbon dioxide with which the air is heavily charged. 

 This gas penetrates a cell wall saturated with water, more 

 readily than do other constituent gases of the atmosphere. 

 In the wall, it is converted into carbonic acid, due to its 

 contact with water, and it then passes into the cell sap 

 of the protoplasts. The quantity of carbonic acid ab- 

 sorbed is determined by the requirements of the cells of 

 particular species. The requirements vary with the time 

 of day. During daylight, all green plants require a 

 quantity of carbon dioxide. Fresh supplies are con- 

 stantly needed, hence the importance of good ventilation 

 in greenhouses. The air should at all times be free from 

 dust or other substances which tend to clog the stomata of 



