CHAP. XXVIII ADAPTATIONS OF WATER-PLANTS 99 



to produce mucilage, or to promote assimilation or respiration ; litho- 

 philous species of algae and Podostemaceae provide examples of the 

 last two cases. 



7. The epidermis or external layer of cells often contains chlorophyll^ 

 and in algae is actually the tissue richest in chlorophyll.^ This must 

 be causally connected with the weakness of light, also with the lack of 

 any necessity for the epidermis to function as aqueous tissue. 



8. Excretion of water is not excluded from submerged water-plants, 

 but when occurrent assumes the form of guttation (the excretion of 

 liquid water) induced by internal activity. At the leaf-tips of many 

 species water-pores occur, or the tips are detached and the ends of the 

 vascular bundles are thus brought into direct contact with the water, as 

 has been shown by the investigations of Sauvageau, Wieler, Weinrowsky, 

 and Minden.2 



Transpiration in the strict sense is of course excluded, and correlated 

 with this is the usual absence of stomata. Where these do occur as 

 exceptions, they may be regarded as functionless vestiges. (In floating 

 plants stomata of abnormal structure occur. )^ 



9. Mucilage is excreted by many water-plants, often in great quanti- 

 ties, sometimes, as in many bacteria and algae, from the general surface 

 of the body, sometimes from special organs, such as hairs in the higher 

 plants, and at other times into internal passages. The function of 

 mucilage has not been definitely ascertained, and possibly may vary 

 widely. In many bacteria, algae, and buds, it perhaps serves as a 

 protection against injury due to rapid physical or chemical change in 

 the surrounding water, and according to the views of Gobel and others * 

 accomplishes this by obstructing the passage of water ; according to 

 Stahl ^ it acts as a defence against animal attack ; while Hunger suggests 

 that it acts as a lubricant facilitating plant-movement. Littoral aJgae 

 living on the shore and lying dry at the ebb, as well as other algae occa- 

 sionally exposed to drying, are protected from desiccation by mucilage ; 

 but in those algae that grow on a rocky shore and are also exposed to 

 the violence of the water, mucilage may serve as a defence against the 

 force of the waves.' Regarding the production of mucilage, reference 

 should be made to the works of Hunger, Schilling,^ Gobel,^ B. Schroder,^" 

 and other authors cited by these. 



10. The chlorenchyma of water-plants is very slightly differentiated ; 

 there is little or no indication of a distinction into palisade and spongy 

 parenchyma in the submerged leaves of Phanerogamia. The leaves are 

 therefore isolateral. This is possibly correlated with weakness of light. 

 Dorsi-ventral structure reveals itself in floating-leaves. 



11. The properties of water bring forth leaf-shapes entirely different 

 from those of land-plants, as will be described in Section IV, which deals 

 with water-plants. The submerged leaves of those plants that also 

 possess aerial leaves are entirely different from the latter as regards both 



' Wille, 1885. 



'Sauvageau, 1889, 1890, 1891, 1894; Wieler, 1892; \\cinrowsky, 1898 j 

 Minden, 1899 ; see Burgerstein, 1904, p. 246. 



' Haberlandt, 1904, p. 413. * Gobel, 1898, 1901. 



' Stahl, 1904 a. " Hunger, 1899. ' ^^ "l^^^' '^^S- 



* Schilling, 1894 " Gobel, 1 898-1901. ' B. Schroder, 1903. 



H 2 



