342 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Another acquired habit is that of climbing by means of the stem. 

 It has often been observed that some genera of plants may have non- 

 climbing species in the open, but climbing ones in shaded places, 

 the climbing habit having been acquired by the seedling being 

 " drawn " when germinating in a dark forest or wood. Then the 

 elongated stem, by circumnutating, cannot help coiling up some 

 neighbouring stem. If a bean or some non-climber with a slender 

 stem, as a periwinkle, be grown in total darkness, and a fine stick be 

 placed close to it, as the stem elongates it will coil spirally around 

 it. This habit can become hereditary. With regard to tendrils, they 

 may be metamorphosed leaves, as in peas, or flowering branches, 

 as in the vine and Passion flower. In the Virginia creeper there are 

 superadded adhesive pads, which are only developed on contact with 

 the wall, to which the hooked tips of the branches of the tendril 

 become attached. In this species of Ampelopsis they are never 

 formed previously to contact ; they are in the Japanese species, but 

 they are not effective till after contact. 



A member of the Cucumber family (Trichosanthes), only known 

 to climb by tendrils which clasp, happened to press its tendrils against 

 the wall of a frame. It at once began to form pads, thus proving 

 its capacity to direct adaptation to a new condition in a new way. 



An ordinary climber may cease to climb if it have nothing to 

 climb up, and assume a creeping habit, as our common bindweed, 

 which will frequently clothe a bank, just as Convolvulus Soldanella 

 spreads over the sand of our sea-shores, but never climbs like the other 

 species of that genus. 



Leaves. — We are all familiar with the diversity of the forms of 

 leaves ; but they are the result of adaptations to the conditions of 

 life. We may roughly divide leaf-blades into broad and narrow. 

 The former are such as can be spread out, so as to be at right angles 

 to incident light ; the narrow ones, as of grass, are the result of growing 

 thickly together, so that they get pretty equally illuminated on both 

 sides, the anatomical structures corresponding, whereas in horizontal 

 leaves the upper and under sides are quite different. Some few plants 

 twist their leaf-stalks — apparently for strength — when the characters 

 of the two sides now become reversed. 



Leaves growing in rather deep water are sword- or grass-like, as 

 those of the Arrow-head. This is due to light falling vertically upon 

 them as on grass in a meadow, as they grow erect under water. 



It is not until the blades can float or rise out of it that they become 

 broad, as in pondweeds and the Arrow-head. The complete identity 

 of form and anatomy between such aquatic and land monocotyledons 

 with those of aquatic dicotyledons proves the descent of the former 

 from the latter, the entire structure being adaptation to an aquatic 

 life, though in terrestrial monocotyledons there have been readapta- 

 tions to the new conditions, without their losing the older aquatic 

 structures to some extent. 



Very rapid changes in adaptation are easily seen in the water- 



