Cladophoreae 263 



The jffigagropila-sectiou of the genus Cladophora deserves special mention on account 

 of the peculiar growth of the thallus. From the very earliest stages there is no differen- 

 tiation into a cauloid part and a rhizoid part, and the plants grow in the form of compact 

 cushions. The lower parts of the thallus (i.e. the oldest segments of the main axis) 

 gradually die, so that the branches are set free from the base upwards. The plants are 

 attached or free-floating, in the latter case forming compact globular masses from 2 to 

 14 cms. in diameter. These floating green balls are often found in great quantity in lakes, 

 and also in sheltered seas, brackish lagoons, and mangrove swamps. They grow very 

 slowly, live for a long time (Scourfield, '08) \ and require very little light. An excellent 

 account of these peculiar Algse has been given by Brand ('02). Wesenberg- Lund's account 

 ('03) of Cladophora (^gagropila] Sauteri confirms many of Brand's observations. He 

 found that much light was prejudicial to growth, and that the compact cushion-like 

 masses resulted from the incessant destruction of the terminal segments of the branches. 

 He regarded the globular shape of the thallus, which so many of these species present, as 

 due to the beating of the waves and the constant friction against the bottom, but it is 

 improbable that this is the full explanation of the ball-formation since these Algso will 

 live in perfectly still water for many years and yet maintain their spherical external form. 



The genus Pithophora is of great interest. It was founded by Wittrock 

 ('77) as the type of what he considered to be a new family of Green Algae, the 

 Pithophoracese. In the vegetative state the plants almost exactly resemble 

 those of Cladophora, and as in that genus the growth of the thallus is 

 apical. The only vegetative distinction between the two genera is in the 

 invariable attachment of the branches of Pithophora some little distance 

 below the apex of the supporting cells, but this insertion of branches may 

 sometimes occur in Cladophora (Brand, '99). The branches are mostly solitary, 

 but are sometimes opposite in pairs. The apical ccenocytes are the longest 

 segments of the thallus, sometimes attaining a length 100 times as great as 

 their diameter ; they may become modified in some species by the develop- 

 ment of terminal claw-like branches. Such apical co3nocytes were termed 

 ' helicoid cells ' by Wittrock, who found them commonly in Pithophora 

 Cleveana but only occasionally in other species. 



The chief character of the genus is the formation of large asexual resting- 

 spores (the ' agamo-hypnospores ' of Wittrock). These are either intercalary 

 and cask-shaped or terminal and ovoid (or even fusiform), and a spore may 

 be formed from almost every segment of the cauloid part of the thallus 

 (fig. 169 A). The spores are usually developed at the upper end of a segment, 

 which in most instances begins to swell, the main mass of the cytoplasm and 

 chloroplasts moving upwards into the swollen part. Soon afterwards a trans- 

 verse wall is formed, appearing first as a ring-shaped septum which gradually 

 becomes a complete separating wall. The cell-wall of the whole spore now 

 grows greatly in thickness. The spores are of an intense green colour and 

 are completely filled by cytoplasm, nuclei, chloroplasts and food-reserves. In 



i The present author has kept these balls alive for over seven years in a small dish into 

 which tap-water was frequently allowed to drop. 



