25] 



CHAPTER I. THE CELL. 



87 



64) are also developed in this way, but in this case the cells are 

 always invested by a cell- wall before they are set free from the 

 mother-cell. A further example of this is to be found in the 

 development of the endosperm-cells in the embryo-sacs of phanero- 

 gamous plants. This mode of cell-formation is" known &sfree cell- 

 formaiion. 



II. Rejuvenescence. The whole protoplasm of the mother-cell 

 may undergo rejuvenescence, when it contracts and reconstitutes 

 itself as the new protoplasmic body of a daughter-cell, which 

 usually does not surround itself with a new membrane for some 

 time. It is in this manner that the single zoospores of many 

 Algse are formed, as in Vaucheria, Stigeoclonium, (Edogonium 

 (Fig. 62), as well as isolated sexual cells such as oospheres. 



Fio. 05. Conjugation of the cells of Spirogyra (x 400). A The cells of two filaments 

 which are prepared for conjugation. At a the filaments have begun to swell towards each 

 other. The spiral bands of chlorophyll are recognisable at cl, and the nucleus at K. At B 

 the protoplasm of the cell p is fusing with that of the other p'. At C is a perfectly-formed 

 zygospore Z. 



III. Conjugation. In conjugation the protoplasmic contents of 

 two or more cells coalesce to form a new cell, which acquires a 

 membrane. This process occurs in a typical manner in various 

 groups of Algae, e.g., Spirogyra (Fig. 65), and of Fungi. 



The formation of new cells does not therefore necessarily imply 

 an increase in number ; this is the case only when division into 

 two, four, or many cells occurs ; in the process of rejuvenescence the 

 number is unaltered, and in conjugation it is actually diminished. 



