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A TEXTBOOK OF THEORETICAL BOTANY 



In addition to true asexual reproduction most species are capable of 

 producing vegetative reproductive bodies termed gemmae which are 

 developed from the tips of the hyphae. They are unicellular bodies, of vary- 

 ing shape, often globose or ovoid, but occasionally quite irregular. They 

 are provided with dense protoplasm and stored food material, and are 

 detached when mature. They germinate by the formation of a hypha which 

 grows into a fresh mycelium. Little is known about the conditions which 

 favour the production of gemmae, but they are partly hereditary, as it is not 

 uncommon to find cultures which persist in forming these structures to the 

 exclusion of any other type of reproductive body. 



Sexual Reproduction 



The sex organs consist of oogonia and antheridia (Fig. 203). The 

 oogonia are borne at the ends of long hyphae, or laterally on short branches, 

 or are occasionally intercalary. They arise in the same way as the zoo- 

 sporangia, by the contents of the hyphae migrating into the tip, after which 

 it is cut off by a septum. The oogonia are generally spherical. The wall 

 of the oogonium is usually smooth but provided with pits, though in some 

 species it is covered with spines or papillae. The contents of the oogonium 

 divide up into a varying number of parts. Rarely one oosphere is formed, 

 but in most species from two to as many as a hundred oospheres may be 

 produced within the oogonium (Fig. 204). These oospheres are spherical 

 and dark in colour, being richly supplied with oil, which serves as a food 

 reserve. This oil is at first found as tiny droplets which coalesce to form 

 a single drop which may either lie in the centre of the oosphere (centric), 

 or may lie to one side of the oosphere (excentric). The oogonium is at 

 first multinucleate and has dense cytoplasm. A central vacuole develops 

 and the cytoplasm and nuclei are pushed out to the periphery, where the 

 nuclei undergo a single mitosis and the majority of the daughter nuclei 

 degenerate. Oospheres now form by the concentration of the cytoplasm 

 around the remaining nuclei, each oosphere enclosing one nucleus (Fig. 205). 



The antheridia may arise either from the same branch as the oogonia, 

 in which case they are said to be androgynous, or from entirely different 

 hyphae, when they are termed diclinous (Fig. 204). The antheridium is a 

 narrow tube containing a rich supply of protoplasm, and is divided from 

 its hypha by a septum ; frequently it may be profusely branched. 



It grows towards the oogonium, and finally becomes closely adpressed, 

 laterally, against its wall. A very fine outgrowth of the antheridium, the 

 fertilization tube, penetrates the wall of the oogonium, generally through 

 a pit, and makes its way to an oosphere. Each oosphere is fertilized by a 

 different antheridium or by a separate branch from the same one. The 

 contents of the antheridium pass into the oosphere through this fertilization 

 tube, and the male and female nuclei fuse. A thick wall is secreted around 

 each oosphere, which thus becomes an oospore (Fig. 207). The oospores 

 are liberated by the breakdown of the oogonium. It seems probable in the 

 case of oogonia with many oospheres that not all of them become fertilized. 



