Chap. I] WATER ^7 



jnly by a portion of its wood, it produces more flowers and eventually 

 norc fruit than an uninjured twig'. Sugar-cane infected with the sereh- 

 lisease having its vessels obstructed by mucilage always blossoms after 

 I short time. 



Moebius has devoted some instructive experiments to the question of the 

 iifluence of moisture on the sexual processes. He cultivated in pots 

 pecimens of Phalaris canariensis, Borago officinalis, and Andropogon 

 schaemum, in sonic cases watering them plentifully, and in others just 

 ufficiently to maintain life. In every case flowering was signally favoured 

 yy drought. Plants that were kept moist did not as a rule produce a single 

 lower during the progress of the experiment. 



To the same group of phenomena belongs Wiesner's observation, accord- 

 ng to which, in saturated air, Capsella Bursa-pastoris bore only a few 

 tunted blossoms and Taraxacum none at all, whereas the vegetative shoots 

 if both these plants developed with extraordinary luxuriance. 



Finally, aquatic plants are highly instructive in this respect. Most 

 quatic phanerogams remain flowerless if a considerable depth of water 

 indcrs the emergence of fertile shoots. Thus, Alisma Plantago, Sagittaria, 

 snardia, Hippuris, Elatine Alsinastrum, Littorella, and others, remain 

 terile whenever they are completely submerged. Many amphibious 

 pecies, such as Marsilea and Pilularia. develop their sporangia exclusively, 

 r nearly so, on their terrestrial forms. Subularia aquatica is cleistogamous 

 hen submerged. On the other hand, vegetative multiplication takes 

 lace in aquatic plants to a very great extent. Thus, in a few years' time, 

 ur waters were overrun by the water-pest Elodea canadensis, developed by 

 leans of its severed branches. 



The Algae, forming by far the greatest class of water-plants, complete 

 leir sexual and asexual reproduction under water. They are plants whose 

 ncestral forms were already aquatic plants, and they have always remained 

 ithdrawn from the influence of drought. Yet in some of the more 

 mphibious Algae the favourable influence of drought in sexual reproduction 

 as been observed ; for instance, by Klebs in Vaucheria. 



Those aquatic plants that have sprung from terrestrial forms, as is the 

 ise with phanerogams and the higher cryptogams, have accommodated 

 lemselves fully to water as regards their vegetative activity ; but, as 

 Jgards sexual reproduction, they have with few exceptions remained 

 ibaerial plants, and this condition has induced wonderful adaptations, 

 Jch as the oft-described pollination of Vallisneria spiralis, which has ever 

 een a theme for poets. Only a few forms, for example Ceratophyllum, 

 faias, Isoetes, a few mosses, and especially the marine phanerogams, pass 

 i^ery stage of their development under water, for which reason special 

 daptations in relation to the liquid environment are induced. Many 

 ' Ernst after Moebius, op. cit. 



