5 36 MICKOSCOPIC FOKMS OF VEGETABLE LIFE THALLOPflYTES 



into two halves (fig. 413, B, a, a'). These then separating from 

 each other, the endoplasm of the parent-cell collects round the two 

 new centres, so as to divide itself into two distinct masses (C, a, a f ) ; 

 and by the investment of these two secondary ' endoplasms ' with 

 cellulose-walls a complete pair of new cells (D, #, a') is formed 

 within the cavity of the parent-cell. The process of free-cell forma- 

 tion is always connected, directly or indirectly, with a process of 

 reproduction rather than of growth, and takes two different forms, 

 the one occurring in the production of the ' zoospores ' or ' swarm- 

 spores ' of alga?, the other in the formation of pollen-grains, or of 



the ' endosperm ' within 

 the embryo-sac of flowering 

 plants. In the former 

 case, the endosperm^ in- 

 stead of dividing itself into 

 two halves, usually breaks 

 up into numerous segments 

 corresponding with one 

 another in size and form, 

 each of which, escaping 

 from the parent - cavity, 

 becomes an independent 

 cell, without any investing 

 cell-wall of cellulose, hence 

 a ' primordial cell,' en- 

 dowed with a power of 

 rapid motion by means of 

 cilia or flagella. In the 

 second case the endoplasm 

 groups itself, more or less 

 completely, round several 

 centres, each of which has 

 its own nucleus, formed by 

 subdivision of the nucleus 

 of the parent-cell ; and 

 these secondary cells, in 

 various stages of develop- 

 ment, lie free within the 

 cavity of the parent-cell, 

 imbedded in its resi<lu;il 

 endoplasm, each proceeding to complete itself as a cell by the 

 formation of a limiting wall of cellullose (fig. 414). As a 'new 

 generation' in any phanerogamic plant has its origin in the 

 fertilisation of a highly specialised ' germ-cell ' (contained within 

 the ovule) by the contents of a ' sperm-cell ' (the pollen-grain), 

 so do we find, among all save the lowest cryptogams, a provision 

 for the union of the contents of two highly specialised cells, 

 the 'germ-cells' being fertilised by the access of motile proto- 

 plasmic bodies (antherozoids), set free from the cavities of the 

 ' sperm-cells ' within which they were developed. But although 

 the sexual process can be traced downwards under this form into 



FIG. 413. Binary subdivision of cells in endo- 

 sperm of seed of scarlet-runner : A, ordinary 

 cell, with nucleus a, and nucleolus b, imbedded 

 in its protoplasm ; B, cell showing subdivision 

 of nucleus into two halves, a and a' ; C, cell in 

 same stage, showing contraction of endoplasm 

 (produced by addition of water) into two sepa- 

 rate masses round the two segments of original 

 nucleus ; D, two complete cells within mother- 

 cell, divided by a partition. 



