322 ALTERNATION OF GENEEATIONS IN THE THALLOPHYTES. 



it is developed from a sexually-iDroduced reproductive cell, the 

 oospore, and that it bears asexuallA'-produced reproductive cells, 

 the s^jores, from which the second generation, the Moss-plant 

 (oophore), is derived : m other words, the spore ahcai/s gives 

 rise to an oox^hore, and the oospore always gives rise to a 

 S2)orophore. 



The following account of the current views on alternation of 

 generations in the Thalloi^hytes is taken mainly from Sachs' 

 'Lehrbuch,"'' where they are clearly and fully stated. Beginning 

 with the Zi/t/osporece, and taking Mucor Miiccdo as an example of 

 the Fungi bi'longiug to that Class, we find that the zj^gospore is 

 regarded as the equivalent of the fertilised oosphere (oospore) of 

 the Moss, and that consequently the product of its development is 

 the homologue of the sporogonium. According to Brefeld's very 

 complete account t of the life-ltistory of this i)l:int, the zygospore 

 gives rise, on germination, to a simple mycelium bearing a 

 single sporangium : this mycelium is then the sporophore of the 

 plant. When the spores (conidia) developed in this sporangium 

 germinate, they usually give rise to mycelia which bear only 

 conidia, and several generations of these may be produced before 

 the recurrence of a sexual mycelium bearing zygospores. It is 

 evident that these conidia-bearing' generations are not the 

 equivalents of rej)eated Moss-sporogonia, for it is the peculiarly 

 modified generation developed directly from the zygospore which is 

 considered to be the sporophore : under these circumstances, the 

 successive conidia-bearing mycelia must be regarded as being 

 patcntiallij sexual, that is, as being capable of producing zygospores 

 under certain favourable conditions, but in the absence of those 

 conditions they produce only conidia. Comparing this life-history 

 with that of a Moss, it appears that the mycelium, developed from 

 the germinating zygospore and never producing Z3'gosj)ores, is the 

 asexual generation or sporophore, and is equivalent to the sporo- 

 gonium : the spores (conidia) which it produces give rise to a 

 generation which, unlike the corresponding generation of the Moss, 

 ina}^ or niav not Ijo sexual according to the conditions under which 

 i^ cxi t.:. 



If we take Panihrina as an example of the algal forms of this 

 Class, we find the siDorophore represented only by the zygospore ; 

 from it, swarm- si^ores are liberated, which are considered to 

 correspond to the s^Dores of a Moss. In this case the mode of 

 development of the spores from the zygospore is much simpler than 

 in Mucur Mucedo; no vegetative organs whatever are developed, but 

 the zygosi)ore is itself the sporangium in which the spores are 

 formed. These two modes of development have been distinguished 

 by Pringsheim as '' wi/celieiiheimunt/ " (Mucor) and '' sponnifiienkei- 

 muiKj " [Fandorina). \ Each of these swarm-spores gives rise to a 

 coenobium, which, like the mycelium of Mucor Mucedo, is either 



* '4te Auflage,' 1874, pp. 231, 238, Av. 

 t ' Bot. Unters. iib. Schininielpilze,' Heft I., 1872. 



I Ueli. den Gen r;iti>Tis\vociisol der Thallophyten etc., ' Jain'.'. I', wis-. Lot.,' 

 Bd. XI., p. A-l. 



