76 STRUCTURAL BOTANY 



shortening of the ordinary life-cycle. The life-cycle is 

 lengthened when we get vegetative propagation of the 

 Fern-plant, so that the number of asexual generations 

 interposed between two sexual ones is increased. This 

 happens in those Ferns which form buds on their leaves ; 

 the buds become detached and give rise to new plants, as 

 may easily be seen in Asplenium lulbiferum and viviparum, 

 so commonly grown in greenhouses. Everybody must 

 have noticed the minute Fern-plants which are dotted 

 about on the fronds of these Ferns, and which in the 

 form of their little leaves are so different from full-grown 

 specimens. 



Another way in which the life-cycle may be extended 

 is by vegetative reproduction of the prothallus just the 

 converse of the process already described. In this case 

 a number of additional sexual generations may be 

 introduced into the life-history. This is pretty common 

 among Filmy Ferns, and in some tropical species, in 

 which the prothallus produces little buds from which 

 new prothalli arise, so that the number of sexual 

 individuals may increase indefinitely without the inter- 

 vention of the sporophyte generation. 



So much for the lengthening of the life-history. In 

 other cases, it is cut short, that is to say, the one genera- 

 tion passes over into the other, without the aid of the 

 regular sexual or asexual reproductive organs. There 

 are two possible cases of this kind; either the sexual 

 generation may give rise directly to the asexual (apogamy), 

 or conversely the asexual generation may give rise directly 

 to the sexual (apospory). We have no space to go into 

 the details of these exceptional modes of development, 

 but it is necessary to mention them, because it is very 

 important to learn at starting that the distinction between 



