158 PLANT MORPHOLOGY 



growth. They may be either loosely or compactly arranged. Some- 

 times they are aggregated to form root-like strands or a compact resting 

 body (sclerotium) . In the development of fruit bodies in the higher fungi 

 — ascocarps and basidiocarps— masses of hyphae become interwoven to 

 form a pseudoparenchymatous structure, but no tissue is formed by cells 

 dividing in three planes. In the lower Phycomycetes the cell wall con- 

 sists largely of cellulose, but in the other fungi its composition is altered 

 by the presence of chitin and other substances, such as fatty acids. 

 Within the cells of the mycelium are one, two, or many nuclei embedded 

 in the cytoplasm. Sugars and glycogen represent the reserve carbo- 

 hydrates, no starch being present. Varying amount of fats may also 



occur. 



Spore Reproduction. The Phycomycetes produce spores in sporangia, 

 either zoospores in the lower orders or aerial spores in the higher orders. 

 The spores are formed in indefinite numbers by cleavage. After escaping, 

 they germinate into a mycelium. The entire sporangium may be persist- 

 ent, as in the Saprolegniales and Mucorales, or detachable, as in most of 

 the Peronosporales. In many of the Ascomycetes and Basidiomycetes 

 the detachable sporangia are replaced by conidia, which function as 

 spores and produce a new mycelium directly. Many conidia, as well as 

 certain other spores, multiply by budding, like the vegetative cells of the 



yeasts. 



Many fungi produce resting spores that are thick-walled and resistant 

 to adverse conditions. Often the same species has two or more different 

 kinds of spores, as in the rusts. Ascospores, which are characteristic of 

 the Ascomycetes, arise by free-cell formation. They are borne internally 

 in an ascus, usually in groups of eight, while basidiospores, characteristic 

 of the Basidiomycetes, are produced externally on a basidium, usually in 

 fours. The formation of ascospores and basidiospores is related to the 

 sexual process. 



Gametic Reproduction. In the Phycomycetes sexual reproduction is 

 alga-like. The Chytridiales and Plasmodiophorales produce free- 

 swimming isogametes that fuse in pairs to produce a zygote. Among 

 the heterogamous Phycomycetes (Monoblepharidales, Saprolegniales, and 

 Peronosporales), all of which have well-developed antheridia and oogonia, 

 only the Monoblepharidales have swimming sperms; in the two other 

 orders a male nucleus reaches the egg by passing through a fertihzation 

 tube. The gametes are nearly always formed within special cells, the 

 gametangia or sex organs. In the higher Phycomycetes (Mucorales and 

 Entomophthorales) the gametangia are not differentiated as antheridia 

 and oogonia, but the entire contents of two gametangia conjugate to form 

 a zygote. 



The Ascomycetes show^ various stages in the degeneration of the sex 



