REPRODUCTIVE CELLS. 149 



tous colony. In one species (^Spirogyra quinina) five adjacent cells 

 conjugate with five others of a parallel filament at the same time. 



(6) Reproduction in Fucus vesiculosus. — Fucus is an alga 

 which to all external appearances seems highly diff'erentiated, 

 there being a system of branching organs which give a false 

 aspect of stem and leaf structures ; internally, however, the 

 histology is seen to be of a simple type, the main tissue being 

 composed of elongated tubular cells joined end to end so as to 

 form an open network. Externally there is, however, a simple 

 type of epidermis, immediately underneath whicli is a zone of 

 small "cortical" elements. The whole plant conforms, however, 

 to the type known as a thallus. 



The organs of reproduction are situated in special parts of tlie 

 thallus, and consist of antheridia and oogonia which arise in spaces 

 known as conceptacles (male and female) found sunk in the tissue 

 of the thallus at the ends of somewhat club-shaped branches. 

 An oogonium arises first of all from a single cell at the bottom of 

 a female conceptacle, and this cell divides into two. The lowest 

 of these is the basal cell, the upper one being the oogonium 

 proper. The contents of the oogonium form the oosphere, and 

 this is a simple mass of nucleated cytoplasm (see Fig. 109, 1, 2, 

 3). The male organ or antheridium arises in the form of a 

 special branching system of tubular cells from the bottom or 

 sides of a male conceptacle. The terminal and a few of the 

 lateral cells of this branch contain cytoplasm and nuclei w hich 

 divide to form the mother-cells of the antherozooids. The 

 antherozooids when mature are set free by the bursting of the 

 wall of the parent-cell as free-swimming motile cells. Each 

 antherozooid is a small nucleated pear-shaped body, possessing an 

 eye-spot, and two laterally situated vibratile cilia (see Fig. 109, 5). 



The oosphere now undergoes a process of maturation. In 

 this process the original cell divides into eight equal-sized 

 egg-cells, each of which is a potential sexual cell. At a certain 

 period the egg-cells are set free by the rupture of the wall of the 

 oogonium, and lie in the conceptacle or in the sea-water in the 

 vicinity of the main plant. Some hair-like structures, the 

 paraphyses, which arise from cells at the bottom and sides of the 

 conceptacle, possibly serve to retain the egg-cells in the chamber, 

 so that occasionally fertilisation may take place in the con- 

 ceptacle itself. 



