3i 



the cell-sap ; these vacuolae extend, and uniting in the centre 

 of the cell, give rise to a large clear space filled with fluid, which 

 is called the cell-vacuole ; as the cell increases in age, the 

 endoplasm becomes changed and used up in the process of 

 nutrition, so that in the cell in age we find nothing but the 

 cell envelope, perhaps a lining of ectoplasm, and a central space 

 filled with cell-sap. 



It is by means of this cell-sap that the plant can take up from 

 outside the fluid substances required for the nutrition of the 

 protoplasm. It usually contains dissolved in it a number of 

 substances either used for the growth of the cell, or excretions 

 of the activity of the cell, e.g., sugar, vegetable acids, colouring 

 matters, seen in the cell-sap of Virginian Creeper (Ampelopsis 

 hederacea), which produces the red appearance of its leaves in 

 autumn and winter ; and also in the purple colour of the 

 filaments of the stamens of Tradescantia. Its contents depend on 

 the age and activity of the cell. The cell-sap acts then as a store- 

 house whence the protoplasm may build itself up, and also a 

 reservoir for the products of its disintegration. 



In order that we may obtain a right appreciation of the 

 phenomena presently to be described, it is necessary that some 

 notice of the minute structure, and mode of growth of the cell- 

 wall and of the protoplasm, should precede our account of 

 them. 



Our knowledge of this portion of the subject is due to the 

 patient and accurate investigations of Carl Von Nageli,* who 

 obtained the first generalizations, through an elaborate and 

 painstaking study of the structure and physical properties of 

 starch grains. Afterwards it was applied to the cell-wall in 

 general, and then to the protoplasm. The facts to be briefly 

 noticed constitute what is known in Vegetable Physiology as 

 " Nageli's theory of the structure of the cell-wall, and of other 

 organized bodies." 



Cell-walls, starch grains, and protoplasmic structures, consist 

 in their natural condition, at every point that can be seen even 



•Nageli, Pflanzen Physiologische Untcrsuchungen, Vol. i, Parts I and 2. Nageli 

 and Schwendener, Das Microscop, Vel. II., p. 402, tt seq. 



