658 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



same time, and the series of conjugation-canals thus formed give the appearance of 

 the rungs of a ladder. All the zygotes are formed in one filament, which we may 

 consider as physiologically female, its gametes being relatively passive compared 

 with those of the other (male), which initiate the process, and actively pass through 

 the canals. The relative behaviour of the two threads shows that an influence is 

 exerted by the male on the female cell, the former determining the outgrowth and 

 direction of the tube belonging to the latter, as well as the rounding off" of the 

 female gamete. If the female tube is not put out opposite the male, the former 

 bends round to meet the latter, and if the male cell dies in the middle of the process, 

 the female tube goes on growing indefinitely, and the female gamete does not round 

 itself off. The influence exerted is in all probability a chemical influence, a pheno- 

 menon which seems to occur in connection with the process of the conjugation of 

 gametes throughout the vegetable kingdom (c/. the remarks on pp. 68 and 413). 



The chromatophore of the germinating zygote is formed from that of the female 

 gamete alone, the band belonging to the male gamete gradually disintegrating in 

 the zygote. This is an interesting example of the reduction of the specially vegeta- 

 tive portion of the male cell. 



Zygnemacece. — The cell of Zygnema differs from that of Spirogyra (fig. 25a, m) 

 in its remarkable and beautiful star-shaped chromatophores. There are two of 

 these in each cell, occupying positions equidistant from the centre of the cell. Each 

 possesses a rounded central portion, containing a single pyrenoid, from which thicker 

 or thinner processes radiate in all directions. The nucleus forms a bridge between 

 the two chromatophores. 



Conjugation takes place much as in Spirogyra, the zygote being formed either 

 in the conjugation-canal or in one of the conjugating cells. 



Mougeotiacece. — This family is characterized by the possession of single axile 

 plate-like chromatophores, and by the fact that part of the protoplasm of a con- 

 jugating cell does not enter into the formation of the zygote. 



The chromatophore, which possesses a single row of pyrenoids, can alter its 

 position according to the strength of the light to which it is exposed. In moderate 

 light the plane of its surface is at right angles to the line of the incident rays, in 

 stronger light it places itself in the same plane as these rays, so that they only fall 

 upon its edge. In very strong light it contracts to form an irregular body in the 

 centre of the cell. 



In conjugation the cells of two threads either put out tubes, and form spherical 

 zygotes in the conjugation-canals (Mesocarpus-type), or the two cells bend towards 

 one another, and form a four-sided zygote, one side of which occupies the centre of 

 each cell. The wall of the zygote thus cuts off the two ends of the two cell cavities, 

 so that the zygote appears as if it were surrounded by four empty cells (Stauro- 

 sperTnum-type). These, however, soon break off. 



Oonatonema forms so-called aplanospores in the following way. A cell increases 

 to double its former length, its contents (chromatophore, &c.) dividing into two 

 parts. A swelling is formed in the middle of the cell, into which the two chromato- 



