286 



THALLOPHYTES, 



FORMS CONTAINING CHLOROPHYLL. 



These are all submerged water-plants, the vegetative and reproductive organs 

 of which have a well-marked tendency to clothe themselves with a peculiar cortex. 

 This is especially remarkable in the genus Chara, and it will be described in detail 

 hereafter when that genus is under consideration. It is also very evident in the 

 Ceramiaceae, and but rudimentary in the Coleochaetese, where it is confined to the 

 fruit. Side by side with forms possessing this cortex, there are others, very nearly 

 related, which do not possess it. 



In all the plants belonging to this group the fruit is small in proportion to the 

 thallus which bears it, and the alternation of generations which finds its expression 



in the formation of the fruit is therefore 

 not very clearly marked. 



A. The Coleoch^te^. 



The carpogonium is unicellular with a long 

 trichogyne opening at its apex. Fertilisation 

 is effected by antherozoids which are formed 

 either in special small branches or in the cells 

 of a filament which have undergone division. 

 In the basal portion of the fertilised carpogo- 

 nium there is a cell which grows considerably, 

 and becomes invested by outgrowths derived 

 from neighbouring cells. In the next period 

 of vegetation it gives rise to numerous carpo- 

 spores in the form of zoospores. 



The Coleochaetese^ are small (about 1-2 

 mm.) fresh- water Algae, of a bright green 

 colour and constructed of branched rows of 

 cells, attached in standing or slowly-running 

 water to the submerged parts of other plants 

 {e.g. Equisetum), and forming circular closely- 

 attached or cushion-like discs. Their chloro- 

 phyll assumes the form of parietal plates or 

 of large granules. The name of the genus 

 Ccleochsete (sheath-hair) is due to the circum- 

 stance that certain cells of the thallus bear 

 lateral colourless bristles fixed in narrow 

 sheaths (Fig. 186, A, h). If the phenomena 

 of growth of the diflferent species are com- 

 pared, two extreme cases are seen, connected 

 by transitional forms. The one extreme is 

 formed by C. di'vergens, which', as it developes 

 from the spore, produces first of all creeping 

 irregularly-branched articulated threads; from these spring ascending articulated 

 branches which are also irregularly branched ; the whole thallus does not assume 

 any definite form. In C. pul'vinata, on the contrary, the thallus forms a hemi- 



FiG. 186.—^ an asexual plant of Coleocheete soluta (x 250); 

 B a piece of a similar disc ; the letters a-g indicate the suc- 

 cessive dichotomous branchings of the terminal cells (after 

 Pringsheim). 



Prii\gsheim in Jahrbuch fur wissenschaftliche Botanik, vol. II. p. 



