THE VEGETABLE CELL. 
387 
or by which the different kinds of tissue are formed from the 
original cell. The various modes of the multiplication of cells 
maybe classed under four heads: — (1.) Free cell-formation; 
(2.) Cell-formation by Conjugation ; (3.) The Eenewal or Reju- 
venescence of cells ; and (4.) Cell-division. Of these the last is 
by far the most common ; but the three first are of the utmost 
importance, as throwing great light on what it is that constitutes 
the vital principle of the cell. 
1. Free Cell-formation is of com- 
3 - paratively rare occurrence. It consists 
in a portion of the protoplasm within 
the primordial utricle of a cell becom- 
ing separated, and secreting a mem- 
brane of cellulose in which it becomes 
Eig. 4. 
Two filaments of Bpirogyra 
longata, showing the com- 
mencement of the process of 
conjugation ( x 550). 
Production of the Zygospore in Spirogyra longata; 
A, an earlier : B, the final stage. 
enveloped, one or more cells being 
thus formed within the parent-cell. In 
this way are formed the “ embryonic” 
or “ germinal vesicles 99 within the em- 
bryo-sac, as well as the first cells of the 
endosperm or “ albumen ” in the ovules 
of Angiosperms ; and the spores within the asci of some species 
of Ascomycetous Fungi, as Peziza. 
2. Cell-formation by Conjugation is- also an exceptional 
phenomenon, being exhibited only in the reproduction of some 
of the lower classes of Alg.se — the Palmellacese, Desmidiacese, 
Diatomacese, and Zygnemacese (sometimes collected together 
into the single group of “ Conjugatse”), and in a few genera of 
Fungi, as Syzygites and Mucor* A very good instance of 
c c 2 
