272 



PLANT LIFE 



like 



in form but unlike in behavior. This leads to the com- 

 pletest form of conjugation, as seen 

 in Mesocarpus, Spirogyra, and other 

 Conjugate. (See *[ 25.) In these 

 the contents of one cell of a filament 

 enter those of another either by a 

 partial solution of the partition-wall 

 between them or by the formation of 

 a tube-like outgrowth from one or 

 both of the cells concerned, so that 

 when these tubes come in contact and 

 have their ends absorbed the contents 

 of one cell passes over into that of 

 the other (fig. 303). The cells con- 

 jugating in this way may he either 

 neighboring cells of the same fila- 

 ment or cells of different filaments 

 brought into proximity by acccident. 



Fig. 303 —Conjugation of .S> iro- , 



gymquinina. The cells a, a' In the course of development in this 



are just forming the conjugat- . . , . 



ing tube; the contents not yet direction conjugation reaches its 

 fully reorganized as gametes. _ i ■ i 



The body protoplasm is not highest perfection, being secured with 



shown in these two cells, ... . , . 



though it is in the others such certainty that non-sexual methods 



(compare fig. 25, of another . 



species of Spirogyra). The are almost entirelv abandoned. 



cells b, b' have completed the «.„„_,, „, 



tube ; the ends have been dis- 376. 2. Heterogamy. — i he second 



solved and the contents of b 



is passing over into b' . This lj ne of development Was followed In- 

 process is nearly completed in 



cells c, c'. 2, 2, zygotes, with other algae, and the method proved so 



protecting wall, thereby pre- ... , - 



paredto become resting spores, efficient that it became the dominant 



Magnified 150 diam. — After 



Strasburger. one in the plant kingdom. Among 



these algre there occurred a differentiation of the zoospores. 

 The first step in this differentiation was an increase in size 

 of one of the sex cells, so that they differed both in action 

 and in form. To distinguish one from the other the larger 

 sex cell is called the female cell, or egg, and the smaller, the 

 male cell, or sperm. A further difference arose in the com- 



