98 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, iii 



organs among Cryptogamia. Some vascular plants, such as Salvinia, 

 Wolffia, Ceratophyllum, Utricularia vulgaris, Aldrovanda, and Genlisea, 

 are entirely rootless ; in others, such as Azolla, Lemna, Hydrocharis, 

 Pontederia, and Pistia, the roots soon cease to grow, do not branch, 

 and may shed their root-caps. Root-hairs are absent in Lemna minor 

 and L. trisulca, Myriophyllum, Butomus umbellatus, Caltha palustris, 

 and Hippuris vulgaris {' except at the collar '), Nymphaea alba, and 

 others.^ 



Roots and root-hairs in many cases are merely anchoring-organs.^ 



2. Water-carrying tubes are for the same reasons in less demand ; 

 wood-vessels and the whole xylem are consequently reduced in vascular 

 plants. Phloem, as the tissue conducting protein bodies, undergoes no 

 reduction. The conducting tissues are always congregated more towards 

 the centre of the organ, so that they finally constitute a central bundle. 

 Van Tieghem establishes four types of degenerate roots. ^ The ramifica- 

 tion and number of veins in the foliage leaves is less than in land-plants. 



3. Mechanical tissue is either reduced or undeveloped because the 

 buoyancy of water is greater than that of air. In particular, those struc- 

 tural designs adapted to resist bending are not developed. In order to 

 resist stretching, due to movements of the water, large fixed water-plants 

 living in very troubled water have their mechanical tissue collected as 

 closely as possible to the centre of the stem so as to form a design adapted 

 to resist tensile stresses * ; while certain algae have strengthening rhizoids 

 at the base of the thallus, as Wille ^ has demonstrated in detail. Lignifi- 

 cation occurs to little (in the wood-vessels) or to no extent. Among 

 submerged water-plants there are no woody plants. 



4. Air-containing spaces are very abundant and large in submerged 

 water-plants and marsh-plants, and serve partly to decrease their specific 

 gravity (as a flotation-device), and partly to facilitate gaseous inter- 

 change and especially respiration. In a number of the larger algae 

 such as Fucus vesiculosus, Ascophyllum nodosum, Halidrys siliquosus, 

 Sargassum, Macrocystis and other Laminariaceae, well-developed flota- 

 tion-devices occur. Exceptions to these statements are provided by 

 nearly all lithophilous hydrophytes, including the vast majority of algae, 

 mosses, Podostemaceae, as well as by some small Spermophyta such as 

 Bulliarda aquatica. 



5. Secondary growth in thickness takes place only exceptionally in 

 the axial organs of water-plants. This is correlated with the matters 

 discussed in the three preceding paragraphs. On the contrary, sub- 

 merged parts of stems and leaves of Spermophyta are far longer and 

 thinner than are the corresponding members when developed in contact 

 with air, and owing to weakness of illumination they almost acquire 

 the appearance of etiolated parts. 



6. The epidermis or the external cell-wall in contact with water is 

 thin, and the cuticle very thin or wanting. Coatings of wax and cork 

 are absent. In contact with air hydrophytes wither and dry up with 

 extreme rapidity. Hairs are lacking from the assimilating organs of 

 nearly all submerged Spermophyta, and when present may serve either 



^ F. Schwarz, 1888. . ^ See Henslow, 1895 5 Warming, 1881-1901. 



3 Van Tieghem, 1 870-1. * Schwendener, 1874. .^ Wille, 1885. 



