628 LECTURE XXII. 



Occasionally it happens that a portion only of the spore 

 gives rise to the embryo. This is the case in the oospores of 

 the Characeae and of the Coniferae. In Selaginella and in the 

 Angiosperms one half of the oospore gives rise to a filamen- 

 tous structure, the suspensor, the other half to the main body 

 of the embryo. 



In many plants the same individual does not produce both 

 sexual and asexual reproductive organs, but they are borne by 

 more or less completely distinct individuals. When this is the 

 case the life-history of the plant includes at least two genera- 

 tions, one of which is sexual and the other asexual ; that 

 is, it exhibits what is known as alternation of generations. 

 Such a life-history cannot, of course, be traced in those- 

 plants, already enumerated, in which the reproduction is 

 effected solely by either sexually produced or asexually pro- 

 duced spores, nor in those, to be now enumerated, in which 

 the same individual bears both sexual and asexual repro- 

 ductive organs. 



The following are plants in which both sexual and asexual reproduc- 

 tive organs are borne by the same individual : 



Algce: Vaucheria; Hydrodictyon ; Ulothrix ; Oedogonium ; some 

 Florideae (e.g. Polysiphonia variegata}. 



Fungi: Many Mucorini ; most Peronosporeae and Saprolegnieae ; 

 Monoblepharis ; among the Ascomycetes, the Erysipheae, Eurotium, 

 Penicillium, Nectria : some Uredineae ( Uromyces appendiculatus, Behenis, 

 Scrophularia, Cestri, Puccinia Berberidis). 



The simplest case of alternation of generations is that in 

 which there are but two generations, the one sexual, the other 

 asexual ; the sexually produced spore giving rise exclusively 

 to the asexual or spore-bearing generation (sporophore), the 

 asexually produced spore giving rise exclusively to the sexual 

 generation (oop/wre). A typical instance of this is afforded 

 by the life-history of Mosses. The sexual generation of the 

 Moss is the moss-plant ; its body is differentiated into stem 

 and leaves and it bears the sexual reproductive organs, the 

 antheridia and archegonia. The oospore, which is formed in 

 the archegonium in consequence of fertilisation does not give 

 rise to a moss-plant, but to the structure which is known as 



