458 FRUITS EEPLACED BY OFFSHOOTS. 



some dozen Mosses. The latter have almost entirely ceased developing fruits, 

 and propagate themselves for the most part by thallidia, since these can be pro- 

 duced much more simply, and their production is independent of enduring drought. 



Certain Ferns must be mentioned in this connection, on the prothallia of which 

 offshoots arise instead of normal, sexually produced embryos. It is true that 

 they form archegonia, but they are abortive, and propagation is asexual. The 

 little Fern-plant arises not from the archegonium but from the tissue in its im- 

 mediate neighbourhood; the archegonia remain closed, are not fertilized, turn 

 brown, and die. This substitution may be observed in AspidiuTn falcatum, in a 

 crested variety of Nephrodium Filix-Tnas, and in the variegated form of Pteris 

 Cretica, frequently cultivated in greenhouses. On the prothallia of normal forms 

 of Nephrodiurii Filix-mas, and on those of wild plants of Pteris Cretica, fertiliza- 

 tion takes place in the usual way, so that it is possible that the substitution of 

 offshoots for fruits is a result of the conditions of cultivation. To what causes 

 exactly the phenomenon in question may be due, is, however, unknown. 



As factors in promoting a substitution of offshoots for fruits amongst the 

 Mosses, other climatic conditions exert considerable influence. But it would lead 

 us too far were we to treat of all these in detail; only a few of the fifty or so 

 examples from the European Moss-flora can be mentioned here. Leucodon sciur- 

 oides, a Moss which rarely fruits in Northern Europe, produces instead numerous 

 leafy shoots (brood-bodies) which, becoming detached, readily root on a moist 

 substratum (see figs. 196^ and 196'", p. 23). Gampylopus fragilis, again, scarcely 

 ever produces fruits in the Alps; it forms readily separable lateral branches, the 

 leaves of which are carried away by the wind. Any of these leaves falling on a 

 moist spot develops green filaments, upon which little buds arise, originating 

 new leafy Moss-stems (see fig. 196'', p. 23). The case of Barbula fragilis and 

 Timmia Norvegica, growing in the Alps, is similar to that of Gampylopus. Of 

 several Mosses the fruits have never been seen; such are Dicranodontium aris- 

 tatum, Barbula papillosa, Grimmia torquata, Bryum concinnatum, and B. 

 Reyeri. They are able to maintain themselves in spite of this by vegetative propa- 

 gation. 



In addition to the cases already enumerated, in which climatic conditions, 

 excess or lack of water, &c., promote vegetative as opposed to sexual reproduction, 

 numerous others are known in which peculiarities in the structure of the flowers 

 cause the ovaries to abort, or make it necessary that a formation of offshoots 

 should be initiated if the plants are to be maintained. In this connection certain 

 hybrid Fuller's Thistles and Mulleins {Girsium and Verbascum) must be noted. 

 The plants in question are hybrids, that is to say, they are produced by crossing of 

 different species. They flower early in the summer, and have ample time to 

 ripen seed before the on-coming of winter, but in a number of these hybrids, owing 

 to variations in the structure of the flowers and of the pollen, few or no seeds 

 are ripened. On the other hand, just these very plants form aerial buds and 

 subterranean offshoots very freely. Girsium purpureum, a hybrid between 



