146 DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



characteristic vegetative body, the chief duty of the adult form is to 

 produce the sexual organs, and it appears as a short-lived appendage of 

 the juvenile form. In consequence of this the juvenile form may acquire 

 so much of an independent character that it develops special propagative 

 organs, from which of course juvenile forms only are again produced. We 

 shall presently see that in some of the Coniferae the 'juvenile form' may 

 even go the length of forming sexual organs, and we have then before us 

 plants in which a section of the development which elsewhere appears as 

 the ' adult form,' and is of great importance for the specific character, 

 appears to be entirely excluded. As examples of plants in which the 



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FIG. 86. 



FIG. 85. Leieunia. Germination of the spore. A flat cellular 'pro-embryo' with a two-sided apical cell is 

 produced. When it has reached a limited size the leafy plant develops from its apical cell. 



FIG. 86. Lejeunia Metzgeriopsis. The 'pro-embryo' is the special vegetative body, and upon it the leafy shoots 

 appear as small appendages, the only function of which is to support the sexual organs. Not so highly magnified as 

 Fig. 85. 



juvenile form has a prolonged existence, whilst in allied forms it is only 

 a developmental stage rapidly passed over, I may note here only two 

 members of the Bryophyta which exhibit the phenomenon in a specially 

 simple manner. Lejeunia Metzgeriopsis (Fig. 86) possesses a richly- 

 branched thallus of a band-like form, which multiplies freely by means 

 of disc-shaped gemmae. This liverwort is nevertheless a foliose one, and 

 the thallus is merely an extended development of the juvenile form which 

 appears in germination of other species of Lejeunia (Fig. 85). The leafy 

 shoots which constitute the characteristic vegetative organs in other 

 members of the genus are only supporters of the sexual organs in Lejeunia 



