THALLOPHYTA. 



657 



Each cell contains a single nucleus which occupies approximately the centre of 

 the cell. In the larger species it is suspended in the middle of the vacuole by a 

 number of branching threads, many of which run into the peripheral protoplasm 

 opposite a pyrenoid. This can be particularly well seen in the large S. crassa. It 

 suggests that the nucleus plays some active part in the function of starch formation. 

 In the smaller species when the breadth of the cell is perhaps not more than double 

 the diameter of the nucleus, the latter often lies in the concavity of, and in contact 

 with, a chromatophore. In this case the protoplasmic threads are not so obvious. 



All the cells of a Spirogyra-threaid are capable of division. After the nucleus 

 has divided, a rim of cellulose is secreted in the equatorial plane of the cell. This 

 is gradually added to from within, till a complete disc of cellulose is formed, sepa- 

 rating the two daughter-cells. Curious folds 

 are always formed on the transverse walls of 

 some species. Each fold forms a circular rim 

 near the periphery of the transverse wall and 

 projecting into the cell cavity. These folds are 

 sometimes completely evaginated, the transverse 

 wall thus having its surface considerably in- 

 creased and becoming strongly concave towards 

 the cell-cavity. This happens especially when 

 two cells are separating from one another, or, 

 when a gamete is formed from a neighbour- 

 ing cell. 



Multiplication is often effected by the break- 

 ing up of a filament into segments consisting 

 of a few cells each which go on dividing and 

 form a new thread. 



Conjugation takes place in two ways; a zygote being produced either from 

 gametes formed in two neighbouring cells of the same filament, or in two cells 

 belonging to distinct filaments. In the former case a small swelling is formed 

 opposite a septum, a small area of the septum breaks down, and the contents of one 

 cell, rounding itself off from the walls, passes through the aperture thus formed and 

 fuses with the contents of the other cell, also rounded ofi", to form a zygote, which 

 immediately puts on a membrane, and enters upon a resting stage. 



In the second or " ladder-type " of conjugation (fig. 373, and fig. 25a, i), two 

 filaments come to lie side by side, and the contents of some or all of the cells of one 

 thread round ofiT, each cell-wall growing out into a short tube towards a cell of the 

 other thread. Each cell of the other thread then either swells up towards this tube 

 or puts out a similar tube, and the walls coming into contact are absorbed, an open 

 canal (conjugation-canal) thus being formed between the two cells. The con- 

 tents of the first cell then passes through the canal into the cavity of the second, 

 the contents of which has also rounded ofi", and fusion occurs between the Uxo 

 gametes. The whole of the cells of two filaments frequently conjugate about the 



Fig. Z73.—Spirogyra. 



1 Two filaments commencing to conjugate, 

 a Formation of zygotes. 



