CHAPTER I. — PROPERTIES OF PROTOPLASM. 221 



i. Contractility, or the power to change its form spontan- 

 eously, or by virtue of forces which reside within. 



2. Irritability, or the power to respond to stimulus, and by 

 reason of which a slight stimulus is capable of releasing a dis- 

 proportionately large amount of energy. 



3. Respiration, which consists essentially in taking in oxygen 

 and giving out carbon dioxide. 



4. Destructive Metabolism, or the changes which take place 

 in the substance of the protoplasm itself, by virtue of which com- 

 plex matters are continually being broken down into simpler 

 forms, and finally into waste materials no longer of service to the 

 plant. 



5. Assimilation, or constructive metabolism ; the power 

 which living protoplasm possesses of taking in new materials and 

 reconstructing old ones for the repair of its waste, or for increas- 

 ing its substance. 



6. Reproduction, or the power which protoplasm has of 

 giving rise to new and similar organisms. 



These properties not only sharply distinguish living from 

 dead matter, but all living protoplasm, both animal and vegeta- 

 ble, possesses them in a greater or less degree. The power of 

 contractility lies at the foundation of all spontaneous movements 

 in animals and plants. Motion is a less conspicuous phenome- 

 non in plants than in animals, but it is no less real. The higher 

 plants show it in the slow movements of all young and growing 

 organs, in the movements of the living matter within the cells, in 

 the bending of organs toward or from the light, or toward or 

 from the earth's centre, and, more conspicuously, in such move- 

 ments as those of the upper internodes of climbing plants, of 

 tendrils, of the leaves of some Mimosas, of Venus' Fly-trap, etc., 

 but they are destitute of the power of locomotion, or of moving 

 from place to place as the higher animals do. On the other 

 hand, while some of the lower forms of animal life are fixed, 

 some of the humblest of plants are conspicuously locomotive. 

 Moreover, the modes of locomotion are, in many cases, identical 

 with those observed among the simpler forms of animals. The 

 Slime Moulds, for example, move from place to place by a slow, 

 creeping process, accompanied by constant changes of form, 

 precisely as in the case of the Amoeba and kindred animal 



