ALTERNATION OF GENERATIONS. 



725 



Structural arrangements, which may perhaps be most easily denoted by saying- 

 that in them the generation which proceeds from the spore remains included in 

 the spore itself, and no longer enjoys independent life, and in the case of the 

 Flowering-plants this generation at length disappears so far, that it is only recog- 

 nisable in its last remnants by careful comparative investigation. 



After these preliminary explanations, which could not well be avoided, the 

 description of a series of examples may now follow. 



Among the Algae and Fungi there are many forms, the whole develop- 

 ment (and especially the reproduc- 

 tion) of which departs from the 

 above type sometimes in one way 

 and sometimes in another. I will 

 select but two examples. Fig. 408 

 shows at OT a portion of the myce- 

 lium of a Fungus, Piptocephalis, pa- 

 rasitic on the Mticor, M, and which 

 has bored into the latter with its 

 haustorium at h. As a rule this 

 mycelium produces conidiophores, 

 as at c, from the ramifications of 

 which small conidia are abstricted 

 in large numbers, and by means of 

 which this Fungus, Piptocephalis, 

 usually propagates itself. Under par- 

 ticularly favourable circumstances, 

 however, a second kind of repro- 

 ductive organs is developed, which 

 we may distinguish as sexual, since 

 the chief mark of sexuality lies in 

 that the contents of two cells fuse 

 with one another, in order to produce 

 a product capable of development, 

 whereas each cell by itself would be 



mCapable of this. This is also true i-'ig. 408.— /■iJlfo^/Ao/li Frts^maua (after BrereU). M a portion 



r .\ nj • » rt. °^ ^^^ mycelium of Mucor Mucedo by which the mycelium mm of the 



01 Ine SO-Callea conjugation Ol trie ^>>fc«//Sai,i is nourished ; a the haustoria of the latter penetratimt 



T-. . . . into the liyphx of the Muirtjr; t a conidiophore ; jj two conjugating 



rUngUS in question: two apprOXl- branches of the myceUum, forming the zygospore Z. 



mating or perhaps mutually touch- 

 ing branches of the mycelium, ss, swell up considerably, become densely filled 

 with protoplasm, and, after a transverse division has occurred in each, their apices 

 come in contact and fuse, and the separating walls dissolve, whereupon the fused 

 portion swells up into a relatively large sphere Z, which becomes segmented off from 

 the conjugating branches jj as a special cell filled with protoplasm, and forms a 

 thick prickly envelope. This so-called zygospore — or, following a new terminology, 

 zygote — requires, like most sexually produced reproductive cells of the Algag and 

 Fungi, a long resting period, before it germinates and produces asexual conidio- 

 phores, whose- spores again give origin to mycelia. 



