218 BOTANY PART i 



not only the internal structure of a true plant, but especially the power of repro- 

 duction and of regular development. 



(iii.) Lastly, organisms' exhibit powers of MOVEMENT; they either 

 change their positions bodily, or they bring larger or smaller parts of 

 their bodies into other positions. Since inorganic bodies and dead 

 organisms may exhibit movements, it is only the kind of movement 

 and the means by which it is brought about that are characteristic 

 of living beings. 



In nature the three processes mentioned above, metabolism, 

 development, and movement, usually go on simultaneously. Meta- 

 bolism without movement of the substances concerned is impossible ; 

 development is bound up with metabolic changes and with movements ; 

 and, lastly, movements cannot occur without metabolism. Neverthe- 

 less, we may for descriptive purposes consider the three processes 

 separately, and thus divide Physiology into the following sections : 



(1) The study of metabolism or chemical physiology, which may 



also be termed the physiology of nutrition. 



(2) The study of development or the physiology of form, changes 



of shape, and the mechanism of development. 



(3) The study of movement. 



5. The full vital activity of the plant is only attained when a number 

 of conditions, which may be divided <into internal and external, are 

 fulfilled ( 2 ). The internal causes of life are connected with the 

 protoplasm. Its structure and organisation not only determine that 

 the changes which take place in the organism have a vital character, 

 but that the organism shows specific differences depending on the 

 descent of its protoplasm. Thus the most fundamental condition of 

 life is the presence of a living mass of protoplasm. All other condi- 

 tions of life can be created or removed at will. The protoplasm, on 

 the other hand, cannot be artificially synthesised, and only arises in 

 the organism by the activity of existing protoplasm. 



The protoplasm can, however, only carry on its activity by con- 

 tinual interaction with the surrounding world. The influence of the 

 latter is threefold. It provides the material from which the body of 

 the plant is built up ; it acts as the source of liberating stimuli 

 (p. 216) ; it provides the plant with the necessary energy either in the 

 chemical energy of substances absorbed from without or as vibrations 

 of the ether. 



In the external factors that are of importance for the life of a 

 plant, a distinction must be drawn between the necessary and the 

 inessential factors. Indispensable conditions of vital activity are a 

 certain temperature and the presence of certain substances, as well as 

 the absence of others that act injuriously or fatally (poisons). On the 

 other hand, light is not in such a general sense a necessary condition 

 for life. Some plants require direct sunlight, at least for their 

 aerial organs, while others avoid this and seek the shade (shade 



