14 



DEVELOPMENT OF CELLS. 



Fig. 44. 



cells are also produced by the division of the primordial utricle, 

 which gradually folds iawards about the middle, forming an annular 

 constriction, and ultimately a complete separation of the utricle into 

 two parts. Each of these afterwards becomes covered by a permanent 

 cell-wall. This is seen in Palmella (fig. 44). Henfrey has supported 

 this view by observations made on the hairs of 

 Tradescantia and of Achimenes grandiflora, in which 

 he has traced the gradual formation of a septum, 

 linger traces in Algse the development of new cells 

 by a fissiparous (fissus, split, and pario, I produce) 

 or merismatic (,u,i§ie//,6g, division) separation of the 

 old ones into two or four divisions, in the same 

 way as occurs in pollen. In some of the most simple plants, multi- 

 plication takes place by a sort of sprouting of new cells from old 

 ones, like buds from a stalk : the portion thus shooting out being 

 afterwards separated from the parent plant by a partition. This is 

 seen in Torula, the yeast plant. 



The various theories of cell-development (cytogenesis) may be re- 

 duced to the following : 1. Formation of cells in protoplasm, existing 

 in the interior of a cell ; 2. Formation of cells in protoplasm, not 

 contained in a cell, but isolated ; 3. Formation of 

 cells by merismatic division of the primordial utricle, 

 or protoplasmic lining of the cell ; 4. Formation of 

 I- C cells by a process of budding. Cells are also formed 

 by what has been called Conjugation, or by the union 

 of two cells, which by their mutual action give origin 

 to a third. This is particularly seen in some of 

 the lower Algse, such as Zygnema (fig. 45). 



The formation of cells goes on with great rapidity, 

 especially in the case of fungi. From an approxi- 

 mative calculation, it is found that in Bovista gigantea 

 20,000 new cells are formed every minute. Ward 

 1 ^p5& I Va^S ^^^ noticed a similar occurrence in Phallus impudicus.' 

 \^^JI\f%^ In warm climates, at the commencement of the wet 

 Fig. 46. • season, the production of cells in the higher classes 

 of plants proceeds with astonishing rapidity. 



In connection with the propagation of cellular plants much discus- 

 sion has taken place as to the existence of their germs in the atmo- 

 sphere, which, coming in contact with fluids of various kinds, are said 

 to give rise to diiferent species of fungi, such as Torula, Penicillium, , 



Fig. 44. Umcellular Alga (Palmella cruenta). The cell, a, absorbs, secretes, and forms 

 new cells, by a process of fissiparous division, first into two, & 6, and then into four parts, c. 

 Fig. 45. Two filaments of a cellular plant (Zygtiema), uniting together by means of 

 tubes, p. The plant consists of a filament formed by a series of cells united in a single row. 

 The cells, c c, appear to have different fanctions. Cell, .i, produced by conjugation. 



