TIIK LEAF 



127 



§ 24. How does the leaf actually perform its assimilatory 

 function ? 



The above-mentioned albumens in various proportions make 

 up a portion of the protoplasmic substance which entirely fills 

 the young (meristematic) cell (Fig. 19), But very soon there 

 arise within the protoplasmic mass which fills up the cells 

 very small quantities of liquid substance (cell sap), which 

 collect together to form small vesicles (vacuoles), and these 

 give to the protoplasm a foam-like structure. Now, as the 

 protoplasm becomes used up during the elongation of the 

 cells, the vacuoles increase in number, run together, and force 



^«; 



H'iG. 19.— Young (Mkki- 

 STEMATic) Cells with 

 Nuclei. 

 a, nucleus; h, nucleolus. 



Fig. 20. — V'oung Paeenchymatous cells. 



the nucleus {cytoblast) (Fig. 20, z) and the remainder of the 

 protoplasm outwards towai'ds the cell- wall. The nucleus 

 which is visible in almost all cells is intimately connected with 

 the life of the cell, and plays an important part in the division 

 of the cells. It has the appearance of a dense albuminous mass, 

 in which one or more small refringent bodies are present, 

 which must probably be looked upon as reserve material, and 

 which are termed nucleoli (Fig. 20, /.). 



With a high magnifying power the nucleus has the ap- 

 pearance of spongy framevvork, or sometimes of a number of 

 loops of thread intimately interlaced, consisting of a denser 

 substance, the chpomatin substance or nuclear thread, the 

 intervening spaces being filled by the nuclear sap or fluid. 



