60 



SIPHONED. 



cilia, and by aplauospores. 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 and 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, /, g) may either germinate immediately, or become a 

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



Order 2. Bryopsidacese. 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, branches and 

 leaf-like structures. The 

 latter have limited growth, 

 and are separated by a cross 

 wall from the stem, and be- 

 come gametangia, or drop 

 off. The gametes have two 

 cilia, and are of two kinds : 

 the female, which are green 

 and large and the male, 

 which are of brownish 

 colour and smaller. Zoospores or anj- other method of asexual reproduction 

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



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

 zoospores, 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 ffidogonium by having a circle of cilia attached at the base 

 of the colourless spot. 



Order 4. Vaucheriacese. 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, 



FIG. 57. Bryopsis plumosa. A the plant, natural size. 

 K A portion (enlarged) which shows the growing point 

 (w), and the leaves derived from it in acropetal succes- 

 sion. 



