12 ZOOLOGY 



parts, to the very much more solid condition apparent in dry 

 seeds and in the resting or encysted stage of some animals. 

 In these latter instances the protoplasm eliminates a large per 

 cent, of its water, forms a thick wall, and thereby becomes 

 enabled to resist drouth and heat and cold as it could not possibly 

 do otherwise. Under ordinary circumstances protoplasm ap- 

 pears as a semi-fluid or gelatinous material. 



Concerning the architecture of protoplasm there is much 

 diversity of opinion. It seems probable that this, like the 

 chemical composition, is subject to considerable variation. It 

 is certainly very complicated and represents several physically 

 distinct substances mingled in a very effectual and wonderful 

 way. Protoplasm is a colloid. This means that the supporting 

 fluid carries in suspension various large molecules or molecular 

 groups. It is not therefore a true solution, and does not pass 

 readily by osmosis as solutions do. In some cases at least it takes 

 take on the appearance of a foam structure such as is obtained 

 in an emulsion of oil in water, or of air and water in a soapy 

 lather. Whatever its form may be, it seems that there must 

 be a close relation between the architecture and the great activity 

 which protoplasm shows. It is physically as well as chemically 

 unstable. 



19. Physiology of Protoplasm. — The mass of protoplasm 

 which we have called a cell, or unit, is able to perform practic- 

 ally all the functions shown by the more complex organism. It 

 has the power of feeding, of growth, of reproduction, of motion 

 in response to stimuli, and of waste and repair. Even in the 

 higher animals, made up of many of these units, the processes 

 are performed, on last analysis, by the individual protoplasmic 

 units of which the body is composed. 



20. Irritability. — Owing to its chemical and physical in- 

 stability, living protoplasm is constantly changing. These 

 changes may be the direct result of internal or external con- 

 ditions to whose influence the protoplasm may respond by a 

 manifestation of energy greater than that involved in the 

 stimulus. This quality is called irritability . It further seems 

 that changes may originate within the protoplasm itself. 



