DEVELOPMENT OF CELLS. 587 



so that the process of cell-division exhibits two principal modifica- 

 tions that in which the new cell- wall is secreted during the divi- 

 sion of the protoplasm, and that in which it is formed only after 

 its complete segmentation. 



This phenomenon may be traced very clearly in all its minutiae in large 

 species of Confer vae (CladopJiora, fig. 593) ; and so far as we may judge 

 from observations, extended from similar cases, through the accessible 

 structures (nascent leaves, prothallia, &"c.) of Mosses, Ferns, &c., up to 

 what we can detect in sections of the tissues of the Phanerogamia, it is 

 the general mode of subdivision of cells. 



The principal varieties which this process exhibits depend on the cha- 

 racter of the tissue to which the dividing cell belongs. In filamentous 

 Confervoids this division takes place in most cases both in the end cell of 

 a filament (apical growth) and in cells forming links further down (inter- 

 calary growth) ; in each case the parent cell elongates more or less beyond 

 the ordinary measure before dividing, and the new cells each grow until 

 they equal the adult length of the parent. In the branched Cladophorce 

 (h'g. 512, C) &c. the parent cell sends out a lateral arm, which is at first 

 a pouch with its cavity continuous with that of the parent ; and this is 

 subsequently shut off by a lateral septum formed in the manner above 

 described. 



The basidiospores of the Agarics &c., and the spores of Penicillium, Bo- 

 try tis, and the allied forms of Fungi, are produced in the same way, as 

 also are the conidia of the " Yeast-fungus/' the new cells emerging like 

 bubbles blown out from the wall of the parent cell, and becoming sub- 

 sequently shut off by a similar process. In the Phanerogamia, the cells 

 of the growing points, as of the apex of buds and roots, of the cambium- 

 layer of the stem, &c., multiply while very minute, so that it is not so 

 easy to trace the changes ; but cell-division may be readily observed in 

 the epidermal hairs of the highest plants, and the protoplasm is observed 

 to be equally efficient as the agent of multiplication in these. The direc- 

 tion in which the division takes place is usually horizontal, sometimes 

 oblique, rarely if ever strictly vertical. It will readily be surmised that 

 the form of the organs and the mode in which they ramify may depend 

 materially on the form of the terminal or apical cells, and on the direction 

 in which they divide. 



The production of complete cells within cells, the septa dividing the 

 new chambers being continuous with new laminae deposited on the old 

 wall of the parent cells, may not only be observed directly in Clndophora 

 (fig. 593, D), but is beautifully proved by allowing filaments of Spirogyra 

 to decay in water ; but these break up into lengths of eight, four, and' two 

 cells, and at last into single short cells, by the solution of the membranes 

 from without inwards. 



The softening and swelling up of these parent membranes doubtless 

 give rise to the semigelatinous coat of many of the lower Algae, espe- 

 cially the Nostochine<s and Palmellece. In the cells of the parenchymatous 

 tissues of the higher plants, their parent membranes are mostly lost sight 

 of by being expanded to extreme tenuity, since the cells here usually in- 

 crease very much in size after their first formation. In woody tissues, 

 formed from cambium-cells, they are mostly so thin as to be almost im- 



