48 



INTRODUCTION TO CYTOLOGY 



- l48t 



which later open up to form vacuoles (Fig. 13). The fixing reagents 

 commonly employed in cytological technique destroy these canalicalce; 

 and since Bensley, by using special reagents, demonstrated such canals 

 in the familiar cells of the onion root tip, it is highly probable that they 

 occur very widely. It seems more reasonable oo suppose that the fluid 



differentiation producos, when they are first 

 forming, gradually come to move along certain 

 paths, forming canals, and later accumulate in 

 the form of vacuoles, than to suppose that the 

 vacuoles originate in such individualized units 

 as the tonoplasts of deVries. 



Fluids other than water may also occur in 

 the form of vacuoles; oil vacuoles, for example, 

 are not uncommon in oertain cells. If fats, oils, 

 and other products of metabolism take their 



Fig. 13.— Cell from root ori g in in chondriosomes, as some suppose (see 

 tip of Allium cepa, showing Chapter VI), it is not improbable that some- 



StoftO* UftCr Cham ~ thing at least anal °g° us t0 the above mentioned 



tonoplast behavior may occur in the case of 



certain substances appearing in the cell. The cell sap and other 



differentiation products in the cytoplasm will be discussed further in 



Chapter VII. 





PROTOPLASM AS THE SUBSTRATUM OF LIFE 



Since the true significance of protoplasm was first recognized in the 

 middle of the last century many suggestions have been ventured regard- 

 ing the nature of the relation existing between life and its physical basis. 

 A full discussion of this subject obviously cannot be entered upon here, 

 but theories of two types, the micromeric and the chemical, may be 

 cited by way of illustration. 



Micromeric Theories. — Many years ago there were developed certain 

 speculative " micromeric theories" of the constitution of protoplasm; 

 these became particularly prominent during the latter half of the nine- 

 teenth century. According to these " atomic theories of biology" the 

 principle of life was held to reside in ultimate fundamental particles. 

 The particles were supposed to be for the most part of ultramicroscopic 

 size, capable of independent growth and reproduction, and associated 

 like members of a vast colony in protoplasm. Such vital units were 

 compared by some to chemical molecules, but they were generally 

 regarded as something much more complex. Examples of such units 

 were the " organic molecules" of Buff on, the "microzymes" of Bechamp, 

 the " physiological units" of Spencer, the " plastidules " of Maggi and 

 Haeckel. the "bioplasts" of Altman, the "vital particles" of Wiesner, 



