332 



REPRODUCTION IN 



togamous plants. A few examples may be adduced, illustrative of 

 the principal modes, beginning with the simplest plants. 



654. Reproduction in Plants of a Single Cell (lOO). All such simple 



one-celled plants as Protococcus and the like (Fig. 79-83, 18 - 22), 

 Desmidiacea3 and Diatomacese, are freely propagated by cell-multi- 

 plication (33 - 36), — the division of their protoplasm or whole living 

 mass into bodies which directly become new cells like the parent, — 

 or by original cell-formation in their interior (29). This is non- 

 sexual reproduction, and essentially answers to the well-known prop- 

 agation of PhfEnogamous plants by buds, bulbs, offsets, &c. It is 

 probable that this may not go on indefinitely in any plant. At any 

 rate, not only do all the higher plants propagate in a different way, 

 viz. by flowers, producing seeds, but probably all plants of the lower 

 grade also have a sexual reproduction in some form or other. It is 

 certainly the case in many one-celled plants, and in others almost 

 equally simple in structure. As in Phasnogamous plants, sexual 

 reproduction essentially depends upon the mingling of the materials 

 of two distinct cells (as the pollen-cell and the embryonal vesicle, 

 579) ; and these cells in the lowest forms of vegetation represent 

 individual plants. The simplest mode of such reproduction in the 

 lowest plants, and that longest known, is what has been termed 



655. Conjugation, This ia the mode in which two vast tribes of 

 microscopic one-celled aquatic plants, the DesmidiaceiE and Diato- 



macece, are reproduced. They midti- 

 ply rapidly, and apparently without 

 limit, by successive division into two 

 equal parts, which separate, each be- 

 coming like the original. But at length 

 two of these individuals, being en- 

 dowed with the power of movement, 

 come into contact ; the firm or often 

 silicious cell-wall ruptures or gives 

 way in a definite manner at the place 

 of junction, and the whole contents of 

 the two conjugating cells or individu- 

 als are commingled into one mass of 

 protoplasm, &c. ; this soon has a coat of cellulose formed around it, 



FIG. 631. Magnified indiyidual of Closterium acutum, after Ralfs. 632. Two individuals 

 more magnified, in conjugation ; ttieir cells opening one into the other, and the contents min- 

 gled ; in 633, condensing ; in 634, collected and formed into a spore. 



