Chap. I] WATER 27 



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

 more fruit than an uninjured twig 1 . Sugar-cane infected with the sereh- 

 disease having its vessels obstructed by mucilage always blossoms after 

 a short time. 



Moebius has devoted some instructive experiments to the question of the 

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

 specimens of Phalaris canadensis, Borago officinalis, and Andropogon 

 Ischaemum, in some cases watering them plentifully, and in others just 

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

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

 flower during the progress of the experiment. 



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

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

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

 of both these plants developed with extraordinary luxuriance. 



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

 aquatic phanerogams remain flowerless if a considerable depth of water 

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

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

 sterile whenever they are completely submerged. Many amphibious 

 species, such as Marsilea and Pilularia, develop their sporangia exclusively, 

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

 when submerged. On the other hand, vegetative multiplication takes 

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

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

 means of its severed branches. 



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

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

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

 withdrawn from the influence of drought.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

 amphibious Algae the favourable influence of drought in sexual reproduction 

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



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

 case with phanerogams and the higher cryptogams, have accommodated 

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

 regards sexual reproduction, they have with few exceptions remained 

 subaerial plants, and this condition has induced wonderful adaptations, 

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

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

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

 every stage of their development under water, for which reason special 

 adaptations in relation to the liquid environment are induced. Many 



1 Ernst after Moebius, op. cit. 



