342 BOTANY 



found to take place in the Phycomycetous group Oomycetes, which are 

 classed on this account nearest the Chlorophyceae. In the Zygomycetes, the 

 second group of the Phycomycetes, and in all the higher Fungi, the asexual 

 spores are non-motile, and invested with a cell wall. This difference 

 is explained by the mode of life. Swarm-spores are produced only by 

 such Phycomycetes as live either constantly or occasionally in water ; non- 

 motile walled spores, on the other hand, are adapted to dissemination 

 by wind, and are accordingly peculiar to the terrestrial Fungi. 



The manner in which such asexual spores are formed shows great 

 variation, and serves as the principal means of characterising the- 

 different groups of the higher Fungi. Two entirely distinct modes of 

 spore-formation may be recognised. 



1 . The formation of endospores within spokangia by the divi- 

 sion of the contents of the sporangia and the production of numerous 

 spores by subsequent contraction (Fig. 270, p. 348). The sporangia 

 are situated, as a rule, at the extremities of special mycelial branches 

 termed sporangiophores. 



2. The formation of CONIDIA (exospores) by the abstriction of 

 spore cells from the ends of elongated hyphae, which are for the most 

 part converted into special conidiophores (Fig. 276, p. 353). Both 

 modes of spore-formation occur in their most primitive form in the Zygo- 

 mycetes, in some cases both methods are represented in the same genera. 

 Transitions between both modes of spore-formation are also observed 

 in certain Zygomycetes, and it would appear probable that a conidium 

 is a more recently developed form of sporangium, and equivalent to 

 a sporangium with one spore. In classifying the higher Fungi which, 

 unlike the Phycomycetes, have lost all indications of sexuality, they 

 may be best treated as derived from the Zygomycetes and divided 

 into two different series. 



In the first SERIES are included the lower and smaller group of 

 the Hemiasci and the higher, more variously modified group of the 

 Ascomycetes. This series, like the sporangia -bearing Zygomycetes, has 

 retained as its principal asexual fructification the sporangium, but 

 elongated and modified into an ASCUS or tubular spore-case. Spores, 

 usually eight in number and arranged in a row, are produced within 

 the asci by free cell-formation (Fig. 273, p. 351). 



The SECOND SERIES, comprising the Hemibasidii and the more 

 highly developed Basidiomycetes, has been derived from the conidii- 

 ferous Zygomycetes. The groups in this series have retained the 

 conidial fructifications, and developed them still further as basidia, 

 or conidiophores specialised in form and size and in the number of 

 their spores. There are various forms of basidia, the most usual 

 being that of the Mushrooms and Toadstools, where four spores are 

 cut off from the ends of a club-shaped support on four slender stalks 

 or STERIGMATA (Fig. 290, p. 368). 



In both the first and second series, in addition to the principal 



