114 XANTHOPHYCEAE 



overlap, their composition being principally of a pectic substance 

 although some cellulose may occasionally be present. The motile 

 bodies contain more than one chloroplast and are further character- 

 ized by two unequal flagellae, the longer one often possessing 

 delicate cilia. The Xanthophyceae exhibit very little regularity in 

 the formation of reproductive bodies. Sexual reproduction is rare 

 and in the few known examples is always isogamous, the principal 

 mode of reproduction being by means of zoospores and aplano- 

 spores. The majority of the species are confined to fresh water. It 

 would seem that they have a motile unicell ancestry, the chief 

 interest of the group being the manner in which evolution has taken 

 place along lines parallel to those found in the Chlorophyceae. 

 As a result there exists a set of analogues which, so far as general 

 morphology is concerned, bear so much resemblance to chloro- 

 phycean groups that these forms are classed as Heterochloridales, 

 Heterococcales, Heterosiphonales and Heterotrichales. 



Heterochloridaceae : Chloramoeba (chlor, green ; amoeba, chang- 

 ing). Fig. 78. 



This is a naked unicell which is analogous to certain members of 

 the Volvocales, e.g. Diinaliella. The cells multiply by longitudinal 

 division, but under adverse conditions ellipsoidal cysts with large 

 oil globules are developed and these form a resting stage. 



Heterocapsaceae : Botryococciis (botryo, cluster; coccus, berry). 

 Fig. 79. 



This fresh-water genus represents one of the palmelloid ana- 

 logues of the Chlorophyceae, the principal species, B. Braunii, 

 forming an oily scum on ponds and lakes in spring and autumn, 

 whilst in late summer the cells are often coloured red by haemato- 

 chrome. The colonies vary greatly in shape, the cells being radially 

 arranged into spherical aggregates that are connected in a reticular 

 fashion by tough, hyaline or orange-coloured strands belonging to 

 the lamellated mucous envelope. The individual cells are surrounded 

 by a thin membrane that becomes evident when they are squeezed 

 out of their envelopes as sometimes happens. Each cell is enclosed 

 in a funnel-shaped mucilage cup composed of several layers and 

 prolonged at the base into a thick stalk. In old colonies the mucilage 

 envelope swells up so that the cup structure is obscured, but al- 



