260 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



I. INFLUENCE OF WATER AND AIR-MOISTURE. 

 A. AQUATIC AND AMPHIBIOUS PLANTS l . 



In aquatic plants the submerged leaves have frequently a different 

 conformation from those which stand above the surface of the water or float 

 upon it, and there are two forms which are specially characteristic 2 

 the riband-form like that of many monocotyledonous plants, and the 

 greatly divided form which occurs in many dicotyledonous plants. 

 Similarly, plants which have the capacity to form two kinds of shoots 

 those which are adapted to a water-life, and those which are adapted 

 to a land-life commonly produce leaves of different form on the 

 different shoots. Superficial observation would lead us to the view 

 that the peculiar form of the water-leaves must be ascribed to a direct 

 influence of the medium. But it is only in relatively few cases that 

 we can prove this, and of course we cannot exclude the probability 

 that in the other cases we have to deal with an inherited adaptation. 

 Besides, it must be remembered that a submerged plant is in conditions 

 of illumination, of temperature, and of nutrition, very different from 

 those of land-plants, and, as has been pointed out in the section upon 

 the influence of light, we know that by feeble illumination Sagittaria 

 may be reduced again to the condition in which it forms the 'riband- 

 shaped leaf typical of the water-leaves under the normal conditions. 

 We have further seen 3 that Sagittaria natans can be caused to revert 

 to the formation of riband-like juvenile leaves by the influence of 

 different factors quite independently of the ' medium.' 



In species of Jussiaea, a direct influence of the medium can be 

 traced 4 . The peculiar spongy breathing roots of Jussiaea grandiflora 

 do not appear if the plant grows upon dry soil. The same may be 

 said of the ' knee-roots ' of Taxodium distichum. 



The configuration and anatomical structure of the leaves of many 

 amphibious plants are quite different according as the plant grows in 

 water or on land. Thus the water-leaves of Ranunculus fluitans are 

 radially constructed, those of the land-form are altogether different 

 and possess dorsiventral lobes 5 . We must consider the latter leaf-form 

 as the more primitive ; it conforms essentially with those of the land- 

 species of Ranunculus. It is also easy to establish in other amphibious 

 species of Ranunculus that the leaves of the water-form have finer 

 and longer lobes than those of the land-form, although the differences 



1 The formation of organs in these plants will be found fully described in my ' Pflanzenbiologische 

 Schilderungen/ ii. ; I give therefore here only a short summary. 



2 See the Third Section, p. 164. 3 See p. 172. 



4 See the figures in my ' Pflanzenbiologische Schilderungen,' ii. 



5 Goebel, Pflanzenbiologische Schilderungen, ii. figs. 97 and 98. 



