BOOK IV. 



PHYSIOLOGY OF PLANTS. 



CHAPTER I. 



THE RELATION OF WATER TO THE PROTOPLASM OF THE CELL. 



We have already seen in considering the structure of- both 

 simple and complex plants, that they are aU composed originally 

 of a variable number of protoplasts, which are in close relation- 

 ship with each other, being partially separated in most cases by 

 septa, or partitions, known as cell- walls. The latter show a 

 great deal of variety in the way they are arranged, their 

 thickness, the material of which they are composed, &c. The 

 protoplasts, on the other hand, resemble each other in all essential 

 particulars, though in complex plants one or other function or 

 property of each is often developed to a greater extent than the 

 remainder which it possesses. 



The essential difference between the protoplasm of the plant 

 and the walls by which it is supported lies in the fact that the 

 former is the living substance, by whose activity all the 

 remainder is constructed. In dealing with the physiology of 

 the plant, we have therefore to turn our attention to the proto- 

 plasm. We have seen that in the simplest forms of plant this 

 may exist without any cell membrane, and may be freely motile, 

 swimming in water by means of cilia, or creeping in a semi- 

 animal fashion by pseudopodia. In other unicellular plants it 

 may be surrounded by a cell- wall, and may either entirely fill 

 the space afforded inside the latter, or may have inside itself a 

 vacuole. In the multicellular plants, each cell while living shows 

 its own protoplast or aggregation of the protoplasm, which is 

 probably connected always with that of the adjoining cells. The 

 protoplasm generally lies as a peripheral layer round the outside 

 of the cell, though in many cases the central cavity is crossed 

 by a number of bridles passing generally from a somewhat central 



