10 THE PLANT CELL. 



Apart from these two main portions, the protoplasm has been 

 supposed to have a somewhat complex physical constitution; 

 some cytologists produce evidence to show that it has a spongy 

 basis, or spongioplasm, which is firm in consistency, and forms 

 a sort of network, in the meshes of which a more fluid portion, 

 or hyaloplasm, exists. On the other hand, Biitschli supposes 

 that it possesses a foam-like structure not unlike that seen in 

 emulsions of clove-oil, bicarbonate of soda, and water ; latterly, 

 however, the idea has been gaining ground that living protoplasm 

 has a quite homogeneous constitution, as careful investigators have 

 failed to detect any special structural basis in it,* whatever may 

 have been observed in preparations of " fixed " and stained 

 l^rotoplasra. 



The chemical composition of living protoplasm is also, as was 

 stated above, somewhat hypothetical ; but one fact is well- 

 established — viz., that the living substance always contains a 

 certain amount of water of constitution. Protoplasm, even of 

 the driest seeds capable of germination, contains this combined 

 water, and once it is removed, either by desiccation or treatment 

 with dehydrating agents, death occurs owing to its extraction. 



There is one property of living protoplasm which completely 

 characterises this substance — viz., its capacity of responding to 

 stimuli, whether these be mechanical, chemical, or produced by 

 light, heat, gra^ it}', or electricity ; a comprehensive term for this 

 property is " irritability," and, as instances of its possession by 

 the living substance, may be cited the following : — ActhaUum, 

 a mass of naked motile protoplasm, when subjected to power- 

 ful illumination withdraws to a position where the light is less 

 intense ; and Amceha, a similar, although, correctly speaking, 

 animal organism, draws in its pseudopodia at once if a harmful 

 stimulus, such as that produced by a crystal of sodium chloride 

 in its vicinity, is brought to bear upon it. 



Fundamentally, there is no essential difference between proto- 

 plasm which is "naked," as in Aethalium, and that Avhich is 

 enclosed within a cell-wall ; in the latter case the living substance 

 may be, and often is, endowed with the power of movement 

 round the enclosing membrane, and light, heat, electricity, and 

 other physical and chemical agencies are found to produce 

 measurable effects when brought to bear upon it. 



* Wilson, The Gel! in Inheritance and Development. 



