Coleoehcetaeece 



07 



or without a single pyrenoid. Zoogonidia and gametes produced in 

 special gonidangia. Sexual reproduction isogamous. 



Family 7. Trentepohliacece. Thallus branched, terrestrial or ar- 

 boreal. Chloroplasts several, parietal, without pyrenoids. Zoogonidia 

 and gametes produced in special gouidangia. Sexual reproduction 

 isogamous. 



Family 1. COLEOCH^ETACE.Sl. 



The plants included in this small family have reached a higher 

 stage of development than any other of the green Alga?, and have 

 undoubtedly arisen from the Chsetophoracea? by further speciali- 

 zation. They form small discs or cushion-like masses, which are 

 enveloped in mucilage and are attached to the stems and leaves of 

 larger water-plants. In the commoner forms the thallus is more 

 or less circular in outline and disc-like in form, consisting of a 



Fig. 16. Coleochcste scutata Breb. ( x 100), from Welsh Harp, Middlesex. 



single layer of cells in one plane, which either form a compel 

 parenchymatous layer, or are arranged in the form of branched 

 filaments radiating from a central point. In other species the 

 ramification is not confined to one plane, but numerous ascending 

 branches are given off, the whole thallus sometimes having the 

 appearance of a hemispherical cushion. The peripheral cells of 

 the disc or the terminal cells of the branches are meristematic and 

 the thallus grows by the formation of new radial and tangential 

 cell-walls. The branching is in some species dichotomous, but in 

 others it closely resembles that of Chcetophora or Myxonema. 



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