204 EXTERNAL CONFORMATION OF PLANTS. 



to Pringsheim)\ or several asexual generations may succeed one another, until 

 finally a sexual generation is produced, as in Vaucheria, Mucorini, and Cystopus. 

 The usual case, and the universal one among Muscineae and Vascular Cryptogams 

 is, however, for sexual and asexual generations to alternate regularly. 



If the morphological conformation of alternate generations is more closely 

 observed, it is seen that in the most simple plants the difference sometimes consists 

 merely in the circumstance that one generation produces asexual spores while the 

 other produces sexual organs, as in Vaucheria and Saprolegnia, where the mor- 

 phological distinctions of the alternate generations are only observable in the 

 preparation for reproduction. But even in the more highly organised Algae and 

 Fungi the alternate generations are mostly very dissimilar in their growth, and the 

 difference is generally especially striking when one generation forms merely spores, 

 the other sexual organs. Thus, for example, the sexual generation in Peziza 

 (where the sexuality has been observed by De Bary, Woronin, and Tulasne) is a 

 thread-like mycelium creeping upon the nourishing substratum, the second asexual 

 generation being developed after fertilisation upon the mycelium in the form 

 of a massive tissue, the receptacle with its numerous spore-sacs. In Muscineae 

 and Vascular Cryptogams, where the alternation of generations is more conspicuous 

 than elsewhere, the sexual generation is always essentially dissimilar from the 

 asexual generation of spores ; and each of the two alternate generations follows 

 an altogether different law of growth. In Mosses, for example, the sexual genera- 

 tion is a self-sustaining Cormophyte, usually lasting for years, and forming on its 

 usually much-branched axis a number of sharply differentiated leaves, root-hairs, and 

 finally the archegonia and antheridia ; while the second generation, resulting from the 

 fertilised ovum in the archegonium, presents itself in the form of a stalked capsule, 

 in which the asexual spores are formed. This so-called Moss-fruit, although it 

 exhibits a marked differentiation of tissue, is a thallus-structure, in which the seg- 

 mentation into axis and leaf is entirely wanting. In Vascular Cryptogams, on the 

 other hand, the sexual generation which proceeds from the spores is a small body 

 of the simplest kind, without any considerable differentiation of tissue, — a thallus 

 which usually never betrays a disposition to any external segmentation or branching. 

 In Ferns, Equisetaceae, and Ophioglossiacese, this organ, called the Prothallium, carries 

 on an independent existence beneath or on the surface of the earth ; in Rhizocarps 

 and the Selaginellee, on the contrary, it remains within the spore. In the female 

 sexual organ (archegonium) of this first generation the second generation arises 

 after fertilisation. In these cases it is always formed into a highly developed 

 Cormophyte, usually possessing unlimited duration of life, and in Ferns often attain- 

 ing large dimensions, and always forming a stem which produces highly organised 

 foliage-leaves, roots, and hairs, and finally the spores in special receptacles. 



1l is one of the most important problems of morphology and systematic 

 botany, not only to demonstrate the alternation of generations in different classes 



^ From the general occurrence of sexuality, and from the circumstance that fertilisation has often 

 been discovered where it was least expected, it must always be doubtful whether even those lowly 

 organised Alg?e and Fungi which we consider at present as altogether asexual do not under certain 

 conditions develope sexual cells. 



