SCIENCE. 



[N. S. Vol. XVII. No. 420. 



confined to them, as most vascular plants 

 develop structures, seeds, tubers, bulbs, 

 etc., which can absDrb water directly. 

 Less commonly the leaves of vascular, 

 plants have this property. This is espe- 

 cially marked in various xerophilous 

 plants, such as the Californian gold-back 

 fern {Gymnogramme triangularis), Selagi- 

 nella rupestris and other species, many 

 species of Tillandsia, etc. 



As all botanists know, the structural dif- 

 ferences between aquatic and terrestrial 

 plants are very marked, but there are some 

 transitional forms which illustrate very 

 beautifully the change from one to the 

 other, and the efforts of the plant to ad- 

 just itself to the changed conditions. Thus 

 some plants which are usually strictly 

 aquatic, such as some water-lilies, may as- 

 sume a nearly terrestrial condition, the 

 long-stalked, floating leaves being replaced 

 by those borne upon shorter upright 

 petioles. 



The primitive aquatic plants are either 

 unicellular or simple cellular plants with 

 relatively little differentiation of parts, as 

 might be expected in organisms living in 

 a relatively uniform medium. A necessity 

 for their active existence is an abundant 

 water supply, as they are not provided with 

 any adequate means for resisting desicca^ 

 tion, although the mucilaginous or gelatin- 

 ous substances in which their cells are some- 

 times imbedded serve to retard for a short 

 time the loss of water by evaporation when 

 they are exposed to the air. A good many 

 of the lower fresh-water organisms are 

 capable of becoming dried up without los- 

 ing their vitality, but of course their ac- 

 tivity is stopped. More commonly they 

 depend upon special resting cells, or spores, 

 to carry them through periods of drought 

 or cold. 



In exceptional cases, the lower algas may 

 assume an amphibious habit, living upon 



wet mud instead of actually in the water. 

 Botrydium and some species of Vaucheria 

 develop a simple root system by which the 

 loss of water by transpiration is made good 

 so long as the soil remains moist ; but these 

 quickly die as soon as the mud dries, as 

 their cells are not protected against loss of 

 water by evaporation. 



It is, however, among the bryophytes, or 

 mosses, that anything approaching a satis- 

 factory solution of the problem of a ter- 

 restrial existence is attained. (I am leav- 

 ing out of account the fungi.) All of the 

 mosses are, to a certain extent, amphibious, 

 since aU of them require first water in or- 

 der that fertilization may be effected. A 

 small number, e. g., Biccia fluitans, Biella, 

 Fontinalis, etc., are genuine aquatics, and 

 the life history of such a form as Eiccio- 

 carpus natans illustrates what has prob- 

 ably been the origin of the terrestrial habit 

 in the primitive archegoniates. Bicciocar- 

 pus is usually a floating plant, but it not 

 infrequently assumes a terrestrial habit, 

 sometimes preliminary to developing its 

 reproductive organs. This is brought about 

 by the subsidence of the water until the 

 plant is left stranded on the sand. Under 

 such circumstances it grows very vigor- 

 ously, develops numerous rhizoids which 

 penetrate the mud and supply it with 

 water. Excessive loss of water is checked 

 by the development of a cuticularized 

 epidermis covering the exposed surface of 

 the thallus. It is highly probable that in 

 some such way as this the algse ancestors 

 of the first arehegoniate plants began their 

 life on land, and slowly emancipated 

 themselves from the necessity of being 

 surrounded by water, and of course thus be- 

 came more and more independent of the 

 drying up of shallow bodies of water in 

 which they grew. In this way the vegeta- 

 tive period would be much prolonged, and 

 would give the plant a great advantage 



