120 



BOTANT. 



252. The reproduction of desmids takes place by fission 

 and by union; that is, asexually and sexually. In the first, 



the neck uniting the two halves of the cell 

 elongates and becomes divided by a trans- 

 verse partition, so that instead of the original 

 Fig. 52.— Ades- Symmetrical cell there are now two exceed- 

 nigh?/ ingly unsymmetrical ones (Fig. 52); these 

 grow by the rapid enlargement of the new 

 and small halves; eventually the two cells become sym- 

 metrical, by which time they have separated. This pro- 

 cess may be repeated again and again. 



253. In the sexual process each of two cells which are 



mid in process of 

 flssion. — ■■ 

 magnified 



Fig. 53.— Sexual reproduction of a desmid (Cosmarium menenghinii). a, front; 

 6, end; c, side view of the adult plants; d, two cells conjugating; e, young rest- 

 ing spore formed; /, ripe resting spore, with spiny wall — the four halves of the 

 parent cells are empty; g, the resting spore germinating after a period of rest; 

 A, the young cell escaped from resting spore; i, young cell dividing, showing 

 two new plants, similar to a, placed crosswise in the interior of the cell. Magni- 

 fied 475 times. 



near one another sends out from its centre a tube, which 

 meets the corresponding one from the other {d, Fig. 53). 

 At the point of meeting the two tubes 

 swell up hemispherically, and finally, by 

 the disappearance of the separating wall, 

 the contents unite and form a rounded 

 resting spore (e), which soon becomes 

 coated with a thick wall (/). After a 

 longer or shorter time the resting spore may germinate, 



Fig. 54. — A common 

 desmid, Closterium. 

 Highly magnified. 



