670 PART IV. THE PHYSIOLOGY OF PLANTS. 



The Products of Metabolism may be classified as plastic products 

 and waste-products: the former are such as can be further worked 

 up in anabolism ; the latter are not so used, but are withdrawn 

 from the sphere of the metabolic activity, by being either ex- 

 creted, or secreted in the insoluble form in special receptacles 

 (see pp. 137 ff ). Of the products of catabolism, carbon dioxide is 

 the most constant. 



3. IRRITABILITY. It is in virtue of the irritability of its 

 protoplasm that the plant is in relation with the external condi- 

 tions under which it is living. Any sudden change in the ex- 

 ternal conditions, or in the direction or intensity of the forces 

 acting upon the plant, evokes some more or less evident response ; 

 that is, it acts as a stimulus, inducing a more or less sudden evolu- 

 tion of kinetic energy. The most striking response to the action 

 of a stimulus is the performance of a movement : it must not, 

 however, be inferred that because stimulation may fail to produce 

 a responsive movement of any part, the protoplasm of that part 

 is not irritable ; for it must be borne in mind that the mechanical 

 conditions may be such as to render movement impossible. 



That it is essentially the protoplasm which is the seat of 

 irritability is shown by the fact that naked masses of protoplasm, 

 such as zoospores and the plasmodia of Myxomycetes, are highly 

 irritable. 



4. AUTOMATISM. Movements may, however, take place without 

 the action of a stimulus, such movements being distinguished as 

 spontaneous or automatic. They are to be ascribed to spontaneous 

 evolutions of energy in the plant, which may be sudden or 

 periodic. 



5. MOTILITY. This property, by means of which the move- 

 ments, whether spontaneous or induced by stimuli, of parts of 

 plants are performed, resides, like the irritability, in the proto- 

 plasm, as is clearly shown by the movements of naked masses of 

 protoplasm. 



6. REPRODUCTION. The function of reproduction consists essenti- 

 ally in the throwing off by the individual of portions of its 

 protoplasm, by which new individuals resembling itself are pro- 

 duced. The reproductive property is generally widely dis- 

 tributed in plants, so that almost any part, if cut off, can develope 

 the missing members, thus completing its segmentation and 

 producing a new individual. In most cases, however, there is 

 provision for the formation of special reproductive cells, termed 



