46 BOTANY 



oxygen left as a waste product. Nevertheless, in every 

 leaf the two processes are going on simultaneously in 

 the same cells at the same time during the day. At 

 night, when it is dark, the carbon assimilation ceases 

 and the process of breathing is not masked, and, con- 

 sequently, the only gas given off by the leaves is car- 

 bonic acid. It is this fact that has led to the old wives' 

 tales that plants are healthy in daytime but poisonous 

 at night. 



Breathing, eating, and drinking are the most vital 

 functions in a plant's life, for if any one of these gets 

 seriously out of order the individual must die. Growth 

 may be arrested, reproduction may be delayed, but in 

 most plants breathing and feeding dare not be inter- 

 rupted for long. In the cases of hibernating animals 

 and hibernating plants, such as our trees when the 

 leaves are off them, there is plenty of food stored in 

 the tissue cells to carry on the passive life of a sleeping 

 organism. 



The plant's responses to the many other stimuli 

 which it is capable of perceiving to a greater or less 

 degree, are generally found to assist it in the main 

 functions of its life. For instance, take the case of 

 the plant's sensitiveness to light heliotropism, as it is 

 called by the professional physiologist. That stems 

 and leaves grow out towards light the geraniums in 

 any cottage window demonstrate. The simple mechani- 

 cal explanation of this bending towards light is that 

 the light actually tends to retard the growth of indi- 

 vidual cells, thus those on the shady side of the leaf 

 stalk grow more quickly, and the whole stalk is con- 

 sequently curved towards the light, carrying the leaf 

 blade with it. This growing towards light is an in- 

 herent character in these parts of plants. It cannot 



