170 ALPINE PLANTS AQUATICS 



tion (e.g. Beech), accompanying which there is a relative increase 

 of spongy tissue, whose intercellular spaces (i.s.} become much 

 more conspicuous. The last-named feature may probably 

 facilitate transpiration in a humid atmosphere. As a general 

 rule the stomata tend to be level with, or even raised above, 

 the epidermis in the shade-form, whilst very commonly more 

 or less depressed in the sun-form. On the other hand, they are 

 generally more numerous in an equivalent area of the latter, 

 the fewer stomata in the shade-form being possibly related to 

 the greater humidity of the surroundings. 



Analogous differences to those exhibited in sun- and shade- 

 leaves are observed between the leaves of plants growing near 

 the tops of high mountains and those found in lowland districts ; 

 this is especially exemplified when the same species occurs in 

 the two kinds of habitats (e.g. Dandelion). In this country, 

 owing to the relatively low height of the mountains and the 

 humid atmosphere at their summits, such differences are not 

 well marked. But elsewhere (e.g. in the Alps) the leaves are 

 commonly thicker and smaller, and have a better-developed 

 palisade tissue, than those of the corresponding lowland form, 

 though they possess a looser texture owing to the large inter- 

 cellular spaces. A rosette- or dwarf -habit is very common (cf. 

 Fig. 221) ; and to this may probably be related the frequent 

 presence of more numerous stomata on the upper, as compared 

 with the under, surfaces of the leaves, since the latter are 

 closely adpressed to the humid soil. 



The relation between anatomical structure and habitat is 

 nowhere more plainly shown than in the aquatics l among Flower- 

 ing Plants, which, however, in many cases betray distinct evidence 

 of their origin from terrestrial ancestors. All the submerged 

 organs of such plants are modified to suit the exceptional con- 

 ditions of the environment. This is well exemplified by a study 

 of the transverse section of the stem of an aquatic like the 

 Hornwort (Ceratophyllum demersum, Fig. 89), which grows com- 

 pletely under water. The vascular system forms a single central 

 strand, as in a root, in correspondence with the fact that here, 

 as there, the main strain is a pulling one. In fact, such a com- 

 pletely submerged plant is everywhere supported by the sur- 



i Cf, F. & S., pp. 331 et seq. 



