456 FRUITS REPLACED BY OFFSHOOTS. 



the flowers for the following summer are already developed in miniature the pre- 

 ceding autumn, so that on the melting of the snow and the termination of winter 

 the flowers can be at once expanded. When such plants can avail themselves of the 

 warmth of the whole summer they are able to ripen fruit and seed. But it is other- 

 wise with those which produce their flowers on a leafy axis, and which must first 

 form an under-structure on which they can be produced; with these, before 

 flowers can be unfolded, a considerable interval of time must elapse. Their blossom- 

 ing is delayed, and the ripening of their seed takes place quite at the end of the 

 period of vegetation. There is thus always the danger of eai-ly frosts or of the 

 winter-covering of snow intervening before the seeds can be ripened and dispersed. 

 It is in just such plants that preservation and propagation are ensured by a 

 development of bulbils; these structures are more speedily produced than seeds, 

 nor do they require so much warmth; further, they are not so liable to injury 

 from premature advent of winter as are developing fruits. The above-mentioned 

 Polygonums, Saxifrages, Rushes, and Grasses are amongst those which flower 

 relatively late, and are liable, in unfavourable seasons, to a destruction of their 

 seeds. The very frequent substitution in them of vegetative for sexual reproduction 

 would seem to be undoubtedly correlated with this liability of seed to fail. And in 

 not a few steppe-plants the substitution of offshoots for flowers is probably con- 

 nected with the fact that with them, also, the season is not always long enough for 

 the formation of stem, flower.s, and fruit. 



It has been previously pointed out that a great many aquatic plants, with 

 roots fixed in the mud and stems and foliage floating in the water, raise their 

 flowers above the surface and avail themselves of the wind and of flying insects 

 for pollination and fertilization. For such plants fluctuations in the level of water 

 must be of considerable moment, and it may well be that if the surface is raised 

 for any length of time, flowering and fruiting are hampered, and, in many cases, 

 rendered impossible. Many marsh and water plants possess, indeed, the capacity 

 of stretching to the surface, the stem continually elongating as the level is raised, 

 until the flowers can be expanded above the surface. But this growth in length 

 has its limits, and it not infrequently lia2)pens that, even after an extraordinary 

 elonsration of stem and flower-stalk, the surface of the water is not attained. And 

 these flowers in most cases cannot be fertilized under water; if already formed, the 

 flower-buds do not open, but atrophj^ and fall oft" without producing fruits. In the 

 little meres of the Black Forest, Littorella lacustris, a plant allied to the Plantain, 

 grows; but it only flowers and fruits in very dry years, when the exi^anse of water 

 is much contracted and the bottom is in large part laid bare. But this is not very 

 often; ten years may pass without the conditions favourable to the flowering and 

 fruiting of Littorella obtaining. During the whole of this time the plant must 

 remain barren were it not for the fact that off'-shoots, wliich take root in the mud, 

 are produced instead of fruits. Thus it is able to maintain and propagate itself. 

 Several Pondweeds and Water-crowfoots (Potamogeton and Banunculus) behave 



