636 SCALE-LEAVES, FOLIAGE-LEAVES, FLORAL-LEAVES. 



investigating the history of plants and attempting to clear up the connection 

 between past and present, recognize from fossil leaves the species living in periods 

 long past, and read the condition of vegetation as it existed many thousands of 

 years ago. Although the results of investigation hitherto obtained in this field 

 are still imperfect, and although these results may receive manifold additions and 

 corrections as more abundant materials come to hand, still the history of vegetation 

 is already exposed in its principal features, and that which has been obtained in 

 this respect during the comparatively short period of half a century is noteworthy 

 in a period of remarkable additions to natural knowledge. In imagination we see 

 replaced the woods and meadows which long ages ago adorned the continents of 

 the Coal period; colonies of slender calamites, the rigid fronds of the cycads, and 

 thickets of countless ferns rise up before us; we are able to sketch landscape 

 pictures of the Jurassic and Cretaceous periods, and to see the banks of the rivers 

 fringed with species of Cinnamomum, evergreen oaks, walnut- and tulip -trees. 

 And all these pictures of the vegetation of the most remote periods would hardly 

 have been possible except on the basis of the determination of species with the 

 help of the minutest investigations into the arrangement and distribution of the 

 strands in the fossil leaves. 



When the leaves of fossil and living plants are compared, we notice that the 

 strands in the former appear more distinct than in the fresh succulent green blades. 

 This is in consequence of the fact that in living plants the strands are often em- 

 bedded in parenchymatous tissue so that they cannot be seen on the surface, while 

 in fossil plant-remains the parenchyma has been wholly destroyed and only the 

 strands have been preserved. When the strands run in the interior of the sub- 

 stance of a leaf, and are not visible at the surface, they are hidden, or, to use the 

 technical term, hyphodromous. Succulent leaves almost always have such hidden 

 strands, which may be contrasted with those which project above the general level 

 on either side of the leaf. On the whole this latter condition is rare, most usually 

 the strands project on one side only, and that the lower surface. Often we find a 

 plexus of ridges on the under side, and one of grooves on the upper side correspond- 

 ing to the course of the strands. The enormous circular leaves of the Victoria 

 regia, which float on the surface of the water, have very strong projecting ribs 

 on the lower side. In leaves of submerged water-plants, however, the strands are 

 insignificant; many are even destitute of vessels, and present only strands of elon- 

 gated cells, as, for instance, the leaves of the celebrated Vallisneria. This is easily 

 understood, as the need of resisting pressure and bending in submerged leaves is 

 very slight. Nor do submerged plants require special conducting tubes for their 

 food-salts. Numerous other striking relations existing between the inner structure 

 of the leaf -blade and the peculiar conditions of the habitat of plants have already 

 been discussed, and we need here merely refer to the description of flattened, 

 rolled, succulent, spiral, arched, hinged, and tubular leaves occurring in the section 

 which begins on p. 209 of this volume. 



The form of the leaf -stalks, stipules and leaf -sheaths, in their dependence on 



