6*2 ORGANOGRAPHY. 



cell, as in the zoospores of the Conferva, &c. In this way- 

 various modifications of this process of cell-division occur, and 

 have been described by authors, some of which are closely 

 analogous, if not referrible to tlie ordinary process of free cell- 

 formation. Our limits will not allow of our alluding further to 

 those modifications. In conclusion we may remark that, while 

 ordinary cell-division is the process by which all vegetating or 

 growing parts are produced and increased, the second process 

 of cell-division, a,nd free cell-formation are especially concerned 

 in the process of I'cproduction. 



By the ordinary method of cell-division, cells are in many- 

 instances produced with almost inconceivable rapidity. Thus, 

 it has been stated that a fungus of the Puff- ball' tribe has 

 been known to grow in a single night, in damp warm weather, 

 from the size of a mere point to that of a large gourd, and it 

 has been calculated, from the average size of the cells in such 

 plants, that sucli a gourd must have contained at least forty- 

 seven thousand million cells, so that they must have been de- 

 veloi)ed at the rate of wcOixXj four -thousand millions per hour, or 

 more than sixty-six millions per minute. It nnist be recollected, 

 however, that this i-apid arowth is not altogether owing to the 

 production of new cells, but also to a great extent upon the ex- 

 pansion of those already formed. Another illustration of the rapid 

 production of cells is afforded us in arctic and alpine regions, 

 where it frequently hapi)ens that the snow over an extensive area 

 is suddenly reddened by the cells of the Red Snow-jdant {Pro- 

 tococcus nivalis) {figs. 147 and 148.) Again, it may readily be 

 ascertained that in a favourable growing season, many stems will 

 increase three or four inches in length in twenty-four hours, and 

 the Agave or American Aloe (Agave americana), Avheu flower- 

 ing in our conservatories, has been known to devolope its flower- 

 stalk at the rate of at least a foot in a day, and in the Avarm 

 climates where it is indigenous, as in the Mauritius, it will 

 grow at least two feet in the same period of time. Leaves 

 also in some cases develope very rapidly ; thus Mulder states 

 that he has seen the leaf of Urania speciosa lengthen at the 

 rate of from one and a half to three and a half lines per hour, 

 and even as much as from four to five inches ))er day. In 

 this rapid growth of stems and leaves, it must be remembered 

 as in the case of tlie growtli of the Pujf'-balt, that such in- 

 crease is due not only to the formation of new cells, but also 

 to the expansion of those previously formed. 



