FRUITS REPLACED BY OFFSHOOTS. 457 



like Littorella, and it would appear that the capacity to propagate by offshoots, so 

 common in aquatic plants, is connected with the impediment to flowering so often 

 presented by a high water-level. Cymodocea antardica, a submerged aquatic 

 plant, which grows in great luxuriance on some parts of the coast of Australia, 

 flowers so rarely that its peculiarly formed bulbils were for a long time regarded 

 as its flowers. Nor has every Botanist seen the flowers and fruits of the Duckweed 

 {Lemna); whilst the renowned American Water- weed, Elodea canadensis, which 

 has been such an obstacle to navigation in canals, &c., but seldom flowers, and owes 

 its very remarkable propagation and disti'ibution, not to fruits, but to a quick and 

 plentiful production of offshoots. 



A dearth of water, also, like a too ample supplj^ can render fertilization im- 

 possible and promote the propagation and distribution of some plants by offshoots 

 to a remarkable degree. In Ferns and Mosses the spei'matozoids reach the arche- 

 gonia, swimming in the water which accumulates on or about the sexual generation 

 of these plants (cf. pp. 65 and 68). In the great majority of cases, it is rain and 

 dew which provide the capillary water which invests the plants, and in which the 

 spermatozoids swim. And other conditions in the life of Ferns and Mosses besides 

 fertilization depend on an adequate supply of water; their existence depends on a 

 certain definite amount and on a certain annual duration of atmospheric precipita- 

 tion. Mosses, and particularly Ferns, have but a restricted distribution in dry 

 localities; or they may be entirely wanting. In humid regions, on the other hand, 

 they attain to a luxuriant growth. The contrast in this respect is striking enough 

 for illustration. Elvend Kuh, a mountain in the interior of Persia, rises to a height 

 of some 3750 metres, and is the culminating point of a considerable plateau. The 

 I'ainy season is limited to a period of two months, and a rich and well-marked 

 steppe-flora covers the ground. Ferns ai'e absent from an area some 5000 square 

 kilometres in extent, whilst Mosses are only represented by a few species which pro- 

 pagate by means of thallidia, rarely maturing spore-capsules. In the hill country of 

 the West Indies, particularly the Blue Mountains of Jamaica, the vapour condenses 

 every moi'ning, and in the course of the afternoon is precipitated as rain. Here are 

 found some 500 Fei-ns, and large numbers of Mosses and Liverworts. The level 

 or sloping ground, rocks, the forest floor and decaying tree-trunks, all are covered 

 with Ferns of every shape and size; there are groves of Tree-ferns, the trunks of 

 trees are invested right up to the crown with delicate, green fronds, whilst tiny 

 representatives of the Filmy Ferns (Hymenophyllacese) have actually taken up 

 their abode on the foliage-leaves themselves. Within a distance of a hundred 

 paces the plant-collector can find fifty diflerent sorts of Ferns, and as many 

 Mosses. 



And between the extremes we have described there are regions with an inter- 



O 



mediate climate, of such a character, that although the fertilization of Ferns and 

 Mosses is not perpetually prevented, still wet years are rare, and several years may 

 elapse without the conditions being favourable for it. Such a region is the Hun- 

 garian plains, the fields and woods of which produce only two species of Ferns and 



