292 STRUCTURAL BOTANY 



from Cryptogams to Phanerogams. In both cases the 

 disappearance of motile male cells is correlated with 

 the loss of aquatic environment. 1 The gradual extinction 

 of zoospores among the allies of Pythium is due to similar 

 causes, and has been fully traced above (p. 222). 



The Zygomycetes, as represented by Pilobolus, are more 

 thoroughgoing Fungi than the Pytliium group, though 

 they still show signs of affinity with Siphoneous Alga3. 

 Perhaps they may have diverged from the algal series at 

 a somewhat different point. 



When we come to the higher Fungi, beginning with 

 such forms as Sphcerotheca, we find it impossible, in 

 the present state of our knowledge, to determine their 

 relation to the algoid forms. The ascus has been 

 regarded as corresponding to the sporangium of the 

 Zygomycetes ; this may perhaps be true, but there 

 is evidence, in cases like Sphcerotheca, for its being a 

 sexually produced sporangium, comparable perhaps to 

 that which is sometimes formed directly on the germina- 

 tion of the zygospore or oospore of the Phycomycetes 

 (see pp. 227 and 234). In any case the Ascomycetes 

 have diverged very widely from the Phycomycete stock, 

 as shown not only by their fructification, but by their 

 septate mycelium. This group reaches a very high 

 development, the ascus-fruit having become a very 

 complex structure in forms like Physcia. The Lichens, 

 most of which are Ascomycetes, are the only Fungi 

 which form a highly-organised aerial thallus. By their 

 association with a green assimilating organism (the 

 captive Alga) they have placed themselves on a level 

 with the higher chlorophyll-containing plants. 



Among the Ascomycetes the conidial form of fruit, 

 though often important, is subordinate. In the remaining 



1 See, however, p. 301. 



