60 



SIPHONED. 



cilia, and by apla.nospores. The sexual reproduction is only 

 known in Botrydium, and takes place in the following manner : 

 in the part of the thallus which is ahove ground arid in an active 

 vegetative condition, several round cells (Fig. 56 c) are formed, 

 which may be green or red according as they grow under water, 

 or exposed to the strong light of the sun.. These cells must be 

 considered as " gametangia " as they produce many gametes (d) 

 provided with two cilia. The zygote (h, i) formed by the conju- 

 gation (e, /, </) may either germinate immediately, or become 

 thick-walled resting-cell of an irregular angular form. 



Order 2. Bryopsidaceae. The thallus in the vegetative condition is uni- 

 cellular, and consists at the 

 lower extremity of branched 

 rhizoids. while the upper 

 portion is prolonged into a 

 stem-like structure of un- 

 limited growth, producing, 

 acropetally, benches and 

 leaf-like structures. The 

 latter have limited growth, 

 and are separated by a cross 

 wall from the stem, and 

 come gametangia, or dro{ 

 off. The gametes have twc 

 FIG. 67. Bnjnvais plumosa. A the plant, natural size. cilia, and are of two kinds : 



B A portion (enlarged) which shows the growing point the femal which are 



(t>), ana the leaves derived f rom it in acropetal succes- , 



8ion . and large and the male, 



which are' of brownish 



colour and smaller. Zoospores or any other method of asexual reproduction 

 are unknown. Only one genus, Eryopsis, living in salt water. 



OrderS. Derbesiaceae. Only one genus, Derbesia, living in saltwater. The 

 y.oospores, which are formed in a few lateral, swollen zoosporangia, possess one 

 nucleus which has arisen through the coalescence of several, and they resemble 

 the zoospores of (Edogonium by having a circle of cilia attached at the base 

 of the colourless spot. 



Order 4. Vaucheriaceae. The thallus consists, in the vegetative 

 condition, of a single irregularly or dichotomously branched cell, 

 without differentiation into stem or leaf ; root-like organs of attach- 

 ment may however occur. Asexual reproduction by zoospores, 

 which are formed singly in the extremity of a branch cut off by 

 a transverse wall. They contain many nuclei, and bear small 

 cilia situated in pairs, which give the appearance of a fine " pile " 

 covering the whole or a great part of the surface. Akinetes, 



