138 VEGETABLE CELLS. 



the parent-cells is partly regular, partly irregular. The 

 arrangement of the secondary cells is regulated by definite 

 laws in vegetative cell-formation, and these produce a 

 definite internal structure and external form of the organ, 

 as 1 have pointed out in another essay,* in Delesseria 

 Hypoglossum, and in the Mosses. In reproductive cell- 

 formation we usually find a regular position connected 

 with a definite number, an irregular arrangement with an 

 indefinite number ; when the germ-cells (in Fungi and 

 Lichens) originate in the constant number of 2, 4, 6, or 8 

 in one parent-cell, they mostly lie in a row in the axis of 

 the parent-cell, or else they exhibit a mode of arrange- 

 ment having a definite relation to this axis. When the 

 special parent-cells are formed in the constant number of 

 four together, they are arranged tetrahedrally about the 

 centre of the parent-cell. But when the cells originate 

 in indefinite numbers (as in the formation of the germ- 

 cells of many Algae and Fungi, and of the endosperm- 

 cells), there is nothing determinate in their relative 

 positions in the parent-cell. 



After the number and arrangement of the secondary 

 cells within the parent-cells, comes the further question, 

 in what way do the particular parts of the parent-cell co- 

 operate in cell-formation? The membrane, as we have 

 seen, in free and parietal cell-formation, bears no imme- 

 diate share in the production of the new cell. It is 

 merely the contents of the parent-ceh 1 which here come 

 into consideration. Normal cell-formation may, in re- 

 ference to the material relation of the parent-cell and 

 secondary cell, be brought into the following categories : 



1 . A secondary -cell originates in a parent-cell, entirely 

 filling up the cavity of the latter, and perhaps including 

 the whole of its contents. Under this head are found 

 the origin of the spore-cells in the special parent-cells of 

 the four-spored Cryptogamia, namely the Floridese, Hepa- 

 ticse, Mosses, Ferns, and Lycopodiacese, and of the pollen- 



* Zcitsch. fur wiss. Botanik. Schleiden und Nageli. Part ii (1845). 



