4 2 PHYSIOLOGICAL MORPHOLOGY 



remarks are intended simply to indicate the physiological standpoint, and 

 to call attention to a few of the more general characteristics exhibited 

 by the protoplast. Neither here, nor in the following chapters, can we 

 discuss the numerous speculations and hypothetical deductions which 

 have arisen in connexion with our morphological .knowledge of the proto- 

 plast, but which are not based on sufficiently sound evidence to satisfy the 

 physiologist. 



Above all, it must be remembered that the simplest protoplast is an 

 organism of very complex structure, and that its various activities result 

 from the interactions of its component parts and organs. The particular 

 result which any given cause produces is due to the special nature of the 

 given protoplast. Every plant must therefore necessarily have certain 

 special protoplasmic characteristics which are peculiar to it alone. At 

 the same time, protoplasts of similar origin may temporarily or permanently 

 acquire special properties by a progressive differentiation of labour, and by 

 adaptation to special aims and purposes. Nevertheless, the plant proto- 

 plast, so long as it remains living, retains all the general features which 

 characterize a typical vegetable cell. 



In order to attain certain ends, the organism forms parts which are 

 not living or capable of life. One such organ is the cell-wall which the 

 protoplast constructs as a protective mantle in which it may live and 

 work; indeed the protoplast living inside its cell-wall may be compared to 

 a snail in its shell. In certain cases, as in Vaucheria, the protoplasmic 

 contents may escape from the cellulose investment as a naked swarm- 

 spore, which later may build for itself a new domicile. 



In the protoplast, just as in a snail, the internal structure and func- 

 tional importance of the component parts require to be studied. Within 

 the protoplast are spaces having considerable functional value, which are 

 surrounded by living substance, but whose contents are not living. 



Such are the vacuoles, which subserve a variety of functions. They 

 may serve for the storage of reserve food material, while the dissolved 

 substances which they contain give rise to the osmotic properties of the 

 cell, and preserve these properties during growth. As the vacuoles increase 

 in size the cell becomes much larger, but the amount of protoplasm which 

 it contains undergoes no increase, or but little, so that finally it is reduced 

 to a thin primordial utricle or bag closely adpressed to the cell-wall, 

 and containing a single large central vacuole. Vacuoles are laboratories 

 in which food may often be digested or building material prepared for 

 use, while at the same time they are utilized in translocation. Fre- 

 quently, as is indicated by their different contents, vacuoles may have 

 special functions to perform, and in such cases the relationships to 

 and interchanges with the vacuoles of other protoplasts are of especial 

 importance. 



