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A TEXTBOOK OF THEORETICAL BOTANY 



The only known method of reproduction is by means of gametes. There 

 is no asexual method of reproduction. 



Sexual Reproduction 



The gametes are not alike and are usually produced on separate plants. 

 The first stage consists of the cutting off of a pinna from the main axis by 



Fk;. 75. — Bryopsis cupressoides. A, Branches of the thallus forming ganietangia. 

 B, Male gametes. C, Female gametes. {After Oltmanns.) 



means of a septum which arises as a ring-like thickening. The whole of the 

 pinna thus forms a gametangium whose protoplasmic contents increase 

 and the chloroplasts multiply by division. In the male gametangium the 

 pyrenoids disappear from the chloroplasts, but they remain in those of the 

 female gametangium. Later the contents divide up by simultaneous cleavages 

 to form gametes (Fig. 75), which are liberated by the gelatinization of the 

 apex of the pinna. The gametes are pyriform with two equal, apical flagella. 

 The female gamete is about three times as large as the male and is provided 

 with a deep-green chloroplast. In the male the chloroplast is yellowish in 

 colour and probably not functional. Gametes fuse in pairs and produce 

 a zygote which may, for a time retain all four flagella. Later it rounds off 

 and germinates directly to produce a new plant. The plants are diploid and 

 reduction division occurs in the formation of the gametes (Fig. 76). 



Vaucheria 



This Alga is largely terrestrial, occurring frequently on damp soils, as, for 

 example, on the soil in flower pots, where it forms a green felt-like covering. 

 It may also be found in fresh water, and it is under these conditions that 

 asexual reproduction most frequently occurs. 



Vaucheria consists of an non-septate, tubular coenocyte with lateral 

 branches. It is dark green in colour and is very sensitive to changes of 

 environment. The coenocyte is frequently attached to the substratum by 



