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 prehminary explanations, which could not well be avoided, the 

 description of a series of examples may now follow. 



Among the Algge 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 m a portion of the myce- 

 lium of a Fungus, Piptocephalis, pa- 

 rasitic on the Mticor, M, and ^hich 

 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 ,c*'5'°| s^ J^'*^</n 11 \W1 }) 



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 

 incapable of this. This is also true 

 of the so-called conjugation of the 

 Fungus in question : two approxi- 

 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 ofi" from 

 the conjugating branches jj as a special cell filletl 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 Algce and 

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

 phores, whose spores again give origin to mycelia. 



l'"IG. af».—Piptocephalis Freseitiana (after Brefeld). M ,t portion 

 of the mycelium of Mucor Mitccdo by which the mycelium m m of the 

 Piftocefhalis is nourished ; h the haustoria of the latter penctratinj; 

 into the hypha; of the Mucor; c a conidiophore ; J- J two conjugating 

 branches of the mycelium, forming the zygospore Z. 



