INTRODUCTION 7 



leaves may or may not have stalks or petioles, and in 

 some cases there are stipules at the base of the latter. 

 The leaves may be compound and made up of leaflets. 

 In such a stage of development grow^th,"^ unless 

 arrested, goes on, and the further stages, such as 

 movements in response to stimuli and special con- 

 trivances for race perpetuation or reproduction, are 

 rendered possible. But all these manifestations of 

 energy are only exhibited if the young embryo be 

 living, and be provided with a suitable supply of water, 

 heat, air or oxygen. Till the food materials reserved 

 in the endosperm or cotyledons are exhausted there 

 is no special call for the other factors, such as light 

 or nutrient salts. But when they are these two 

 factors must be forthcoming in order that absorption 

 and photosynthesis may serve to carry on nutrition, 

 or otherwise growth ceases, and the plant will die or 

 wilt. They are necessary for the performance of the 

 other functions described in vol. ii (Introduction). 



II. The Movements of the Plant. Response 

 TO Stimuli. 



Even in the seedling stage the plantlet responds to 

 two classes of stimuli, those of gravity and light. The 

 terms geotropism and heliotropism are used in con- 

 nection with the response of the plant to these stimuli. 



When, however, a plant has begun to obtain its 

 own food from the air and the soil, or water in the 

 case of aquatics, these classes of stimuli are over- 



* Vide also p. 8. 



