PHTOOPHTTA. 



139 



243. The Desmids {Desmidiacece) are minute unicellular 

 fresh-water plants. The cells are of very various forms, 

 usually more or less constricted in the middle, and divided 

 into two symmetrical half-cells. The cell-wall is more or 

 less firm, but never siliceous. 



244. 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 

 transverse partition, so that instead of the 

 original symmetrical cell there are now two Fio- 69.— A des- 



■^ *^ mid m process of 



exceedingly unsymmetrical ones (Fig. 69) ;^|^JJijg^^'s''^y 

 these grow by the rapid enlargement of the 

 new and small halves; eventually the two cells become 

 symmetrical, by which time they have separated. This 

 process may be repeated again and again. 



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



Fig. 70.— Sexual reproduction of a desmid (Oosmarium meneghinil). a, 

 front ; 6, end ; c, side view of the adult plants ; d, two cells conjugating ; 

 e, young resting spore formed ; /, ripe resting spore, with spiny wall— the 

 four halves of the parent cells are empty ; p, the resting spore germinat- 

 ing after a period of rest ; U, the young cell escaped from resting spore ; t, 

 young cell dividing, showing two new plants, similar to a, placed cross- 

 wise in the interior of the cell. Magnified 475 times, 



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



meets the corresponding one from the other {d, Fig. 70). 



At the point of meeting the two 



tubes swell up hemispherically, 



and finally, by the disappearance 



„ ,T ,. m ii Fig. 71.— a common des- 



of the separating wall, the con- mid, ciosterium. Highly 



magnified. 



tents unite and form a rounded 



