24 



SPORES AND THALLIDIA. 



in a brood-body of the kind is limited to two, as is the case in the so-called 

 " teleutospores " of the Rust-Fungus; whilst those of Florideae sometimes have 

 four cells and are known as " tetraspores ". Again, in other cases hundreds of 

 cells are associated together to form a thallidium, an instance of which is afforded 

 by the brood-body or gemma of Marchantia (see fig. 196 1>2 > 3 ). The "soredia" of 

 Lichens must also be brought under this head by the term soredia being under- 

 stood certain bodies which arise upon the thalli of Lichens and consist of one or 

 more green cells wrapped in a net-work of colourless hyphae (see vol. i. p. 248). 



Fig. 197. Formation of Thallidia in the cells of Hydrodictyon and in those of Pediastrum. 



I Water-Net (Hydrodictyon utriculatum), natural size, a A piece of the Water-Net ; x 50. 8 , *, 6 Development and escape of 

 a reticulate thallidium ; x 300. Pediastrum granulatum ; development and escape of thallidia ; the lightly-dotted cell 

 chambers already vacated. *, 8 Thallidia of Pediastrum after their escape ; x240. 



Thallidia may originate in the interior of a cell-cavity of the parent-plant 

 and escape in the form of complete, though extremely minute, cell-aggregates. 

 Instances of this are afforded by the Water-Net (Hydrodictyon utriculatum), 

 which is shown in fig. 197 5 , and by Pediastrum granulatum (fig. 197 6 ), an 

 organism of frequent occurrence in pools. The alternative method of formation 

 of thallidia is by the severance of groups of superficial cells, which, after an 

 interval of peregrination of variable duration fasten on to some spot or other 

 and found a new colony. In many Liverworts and Mosses special pockets and 



