INTRODUCTION 253 



time demonstrate the great importance of this factor in determining the 

 geographical distribution of the plant. Since certain Algae have their optimum 

 about 0° C, and certain Bacteria at from 60° to 70°, it is manifest that they 

 must necessarily be inhabitants of very different regions. This example shows 

 us that external factors are indispensable conditions of plant development, 

 and that, too, not in the case of the higher plants only but in the case of all 

 organisms. We term these ' formal ' conditions of development to distinguish 

 them from special formative external influences which on closer examination 

 affect the higher plants also. 



On examining plants of different families which live together under one or 

 other extreme, we find that they have one capacity in common, viz. that they 

 can adapt themselves to these extremes (compare Volkens, 1887, Goebel, 

 1889-93, 1, 25 ; 1, 149 ; 2, 3 ; Kerner, 1891, &c.). Thus desert plants, which have 

 difficulty in satisfying their requirements so far as water is concerned, exhibit 

 numerous adaptations for retarding transpiration ; their surface is limited in 

 extent by reduction in the size of the leaves, the function of assimilation being 

 undertaken by the stem (Cactaceae and Euphorbiaceae) ; further their cuticle 

 is thickened, their stomata are deeply seated and they cover themselves with 

 layers of wax or hairs. On the other hand, we find in them many arrangements 

 for bringing about a maximum absorption of water if such be available ; they 

 exhibit an especially extensive and deeply penetrating root-system. 



In marked contrast to these ' xerophytes ' are the ' hydrophytes ' or 

 aquatic plants, especially the submerged types which we shall now consider 

 (compare Askenasy, 1870 ; Schenck, 1886 ; Goebel, 1893, 2, 215). These 

 forms are capable of absorbing water by their entire surface and have nothing to 

 fear from loss by transpiration. Accordingly, the root in these plants is quite in 

 abeyance as a water-absorbing organ; the water-conducting tissue is feebly 

 developed ; the cuticle is thin and easily permeable and the mechanical tissue 

 is often entirely wanting. Submerged plants experience, however, difficulties 

 in gaseous exchange. They can obtain gases only from the surrounding water, 

 and thus, doubtless, we may account for the enormously increased leaf-surface 

 produced by the formation of numerous delicate projections. Roots and 

 rhizomes especially which are imbedded in mud must be supplied with the 

 necessary oxygen from the parts above, and hence may be explained the extra- 

 ordinary development of intercellular spaces which is a feature of all hydro- 

 phytes. Stomata, on the other hand, the normal apertures for gaseous exchange 

 in land plants, are wanting entirely in submerged forms. 



These brief notes may suffice to show that plants adapt themselves to their 

 surroundings. We should be incorrect, however, in assuming that the special 

 shape of the hydrophyte was in any sense induced by the medium in which 

 it lives. We are acquainted with forms, the so-called amphibious plants, which 

 are capable of living both on the land and in the water. We may note especially, 

 in illustration. Polygonum amphibium, whose land and water forms differ remark- 

 ably from each other. When in water, the rhizome is long and obliquely ascending, 

 bearing several leaves with long petioles and heart-shaped, broadly lanceolate, 

 leathery blades floating on the water. The entire plant is smooth and glabrous. 

 When grown on land the stem is erect, the leaves are narrow lanceolate, quite 

 sessile, wrinkled, and partly hairy. The aquatic and terrestrial forms may exist, 

 however, concurrently as two branches of the same rhizome. The aquatic form 

 of Ranunculus aquatilis possesses extremely finely divided leaves and long 

 internodes, the land form has short internodes and broader leaf apices. The 

 anatomical differences between these leaves are especially striking ; those of 

 the land form are rigid, bear stomata, and have their assimilatory tissue dorsi- 

 ventrally arranged ; while those of the aquatic type are soft, have no stomata, 

 and have radially-arranged assimilatory tissue. Still in this plant also it is 

 possible by ctiltivating the terrestrial form in water to turn it into the aquatic type. 



