GROUP I. THALLOPHYTA : ALOE. 



255 



vacuole in which the nucleus is situated in a mass of protoplasm con- 

 nected with the peripheral layer by several delicate protoplasmic 

 filaments. The chloroplastids are the most conspicuous feature of the 

 cell ; in Spirogyra (Fig. 



145) the chloroplastids, of ^ 

 which there may be from /,! \ 

 one to four, lie in the & ^**r*. --.- . .:..-^-^. \oEJ^ 

 parietal protoplasm ; they 



are spirally twisted in 

 Spirogyra, but are nearly 

 straight in Sirogonium : 

 like those of the Desmids, 

 these chloroplastids con- 

 tain several pyrenoids 

 with associated starch- 

 grains ; in Zygnema (Fig. 

 144 A) each cell contains 

 two chloroplastids, sus- 

 pended in the middle line, 

 each containing a pyrenoid with starch-grains. 



The filaments elongate by the growth and division of all the constitu- 

 ent cells, and readily break up into segments, consisting of one or more 

 cells, which grow into new filaments. 



The sexual organs (gametangia) are quite undifferentiated ; any or all 

 of the cells of a filament may act as sexual organs, the whole of its con- 

 tents being converted into a single non-oil iate gamete. The sexual process 

 (conjugation) consists in the fusion of the gametes derived from two cells 

 belonging generally to two filaments, but sometimes to the same filament. 

 It is effected, in most cases, by the development of a lateral outgrowth 

 from the middle of each gametangium ; the two outgrowths meet (Fig. 



146) and their walls become absorbed at the point of contact so that the 

 cavities of the two gametangia are continuous. The protoplasmic con- 

 tents of each cell contract away from the wall of each gametangium to 



FIG. 144. .4 Fragment of a filament of Zypnenia ; 

 in each cell are two star-shaped chloroplastids con- 

 nected by a colourless mass of protoplasm in which lies 

 the nucleus. B Closterium. C Eaastrnm, two Desmids 

 with chloroplastids; in B there is a vacuole at each 

 end in which a number of granules may be seen in 

 motion. 



FIG. 145. Conjugation in Spirogyra ( x 400). At A two cells of adjacent filaments are 

 about to conjugate, and are putting out protuberances (a) towards each other; ct chlcro- 

 plastid ; fc nucleus. At B the gamete p of the one cell is passing over and fusing with 

 that of the other (p). At C the process of conjugation is completed, the zygospore Z being 

 the product. 



