400 ELEMENTARY BIOLOGY. [CHAP. 



distinct halves, each containing one of the daughter nuclei. 

 The whole process, especially as regards the division of the 

 nucleus, is a complicated one, but it is not necessary to enter 

 into the details here. Cell-division, of which this may be 

 taken as a type, is the general mode of formation of cells 

 throughout the vegetable kingdom. In the higher plants 

 however it is usual for the new wall to be formed simul- 

 taneously over its whole area, instead of by progressive 

 growth from without inwards. 



The Reproduction of Spirogyra takes place in quite a 

 different way from that of the plants hitherto considered, 

 and affords one of the simplest examples of a sexual process. 

 In the form in which it here occurs this process is termed 

 conjugation, which may be defined as the union of two ex- 

 ternally similar cells to form a single reproductive cell. 

 Conjugating specimens may be' distinguished from vege- 

 tative ones by their more tangled filaments, and duller 

 colour, and by feeling rougher when handled. When under 

 cultivation Spirogyra can usually be induced to conjugate 

 by allowing the water in which it is growing to evaporate 

 very gradually. In the ordinary form of conjugation two 

 filaments lying side by side send out short lateral protrusions, 

 one from each cell. The protrusions from the one filament 

 grow to meet those from the other, their contiguous walls 

 become absorbed, and in this way the two filaments become 

 connected by a number of transverse tubes, arranged like the 

 rungs of a ladder. Changes now begin to take place in the 

 contents of the cells, and it is at this stage that a difference 

 first shows itself in the behaviour of the two filaments. In 

 the one filament the contents of each cell contract away 

 from the wall, so as to lie freely in the cavity of the cell 

 as a distinct sac bounded by the primordial utricle. In 

 the other filament these changes take place somewhat later. 



