54 MANUAL OF BOTANY 



anatomical and morphological differentiation is but slight, the 

 peculiarities of sexual reproduction are thus more like those in 

 the higher plants than are those of any of the other groups of 

 Algse. 



The plants of this sub-class are, with few exceptions, very 

 simple in the structure of their vegetative body. Like tliose of 

 the last group, the predominant form is the gametophyte, the 

 sporophyte being quite rudimentary, and consisting either of the 

 fertilised female gamete, or of a small body resulting from a few 

 divisions of the latter. They show several types of structure. 

 Many are unicellular, and in that case of microscopic dimensions. 

 Others form filaments, each made up of a single row of pre- 

 cisely similar cells, all of which are physiologically alike. A 

 third type is seen in flat plates of cells, only a single cell in 

 thickness ; these may be very large and leaf-like, or they may 

 form a very small group, microscopic in size. A group of this 

 kind, in which the cells show no differentiation, or at most a 

 difference between the ordinary cells of the thallus and the 

 reproductive cells, is called a coenohiuyn or colony. Sometimes 

 the coenobium is spherical, and its cells are all furnished with 

 cilia, giving it powers of locomotion. In a fourth type the thallus 

 is coenocytic — that is, there are no internal cell-walls formed 

 between the constituent cells, so that the whole plant appears 

 like a single large cell with many nuclei embedded in its pro- 

 toplasm. The coenocyte may be a filament, or a somewhat 

 globular body with a branched base {fig. 804), or a branched 

 structure showing differentiation into stem, and leaf and root. 

 In some cases of coenocytic structure the body is divided by 

 transverse walls in several places, so that it may be regarded as 

 composed of a number of coenocytes. Lastly, the body may 

 be cellular and differentiated into root and shoot, the latter 

 bearing branches and leaves. 



In habit the Chlorophyceae vary but little. They are found 

 in both salt and fresh water. Most forms are free ; some are 

 attached to a substratum. A few are endophytic, inhabiting 

 cavities in other plants much as Nostoc does. They present 

 hardly any histological differentiation, with the exception of the 

 Characeae, where the stem sometimes shows a rather curious 

 structure, and curious reproductive bodies are produced. 



The gametophyte may produce both asexual and sexual cells, 

 and may reproduce itself vegetatively by simple division or by 

 the production of gennnae. The sexual organs are sometimes 

 very simple, any cell of a filament having the power to produce 



