January 1, 1895.] 



KNOWLEDGE 



19 



l.i 



eili 



mi 



Wi;) 



WmWm 



Fu 



Fig. 



III.- -Transverse Seetion of Fibro-vaseular 



Btmdle. 

 IV. — Vertical Section of Fibro-vaseular 



Bundle. 



our assuming that green petals necessarily represent 

 an early stage in the evolution of the corolla. As regards 

 the ivy at least, there is little room for uncertainty ; the 

 inferior ovary and united carpels are certain marks of 

 advanced organization. The closest relationships of the ivy 

 order are with flowers having united petals, especially with 

 the CiiprifuUaci'a, which includes the honeysuckle, elder, 

 snowberry, guelder-rose, and laurustinus, and such i-elation- 

 ship is also indicative of a high degree of development. 



The aborted 

 sepals, again, 

 afford clear 

 evidence of 

 degradation, 

 and when all 

 other parts 

 of the flower 

 show signs of 

 extensive 

 modification 

 it is highly 

 improbable 

 that the pe- 

 tals alone 

 should have 

 retained their 

 primitive 

 ch a racter. 

 The inflores- 

 cence, the 

 floral e n - 

 velopes, and 

 the natural 

 affinities of the plant point unquestionably to ancestors 

 possessed of blossoms more gaily attired than those with 

 which the ivy is now adorned. 



The ovary as it matures develops into a black berry, 

 which ripens in February. Ivy berries are said to possess 

 emetic properties. Baccate fruits of this description, as 

 explained in a previous article, have their seeds dispersed 

 through the agency of birds, which are attracted by the 

 colour and sweetness of tlio fruits. 



The leaf arrangement of the ivy is alternate or spiral, 

 and each leaf is supported on a long foot-stalk. In size 

 and shape the leaves vary very much ; the three-lobed 

 form is frequent, but perhaps the five-lobed or palmatifid 

 is the typical form. This shape is thought by Sir John 

 Lubbock to possess the advantage of bringing the centre 

 of gravity near the point of support. There is a remarkable 

 difl'erence in the ivy between the leaves of the flowering 

 shoots and those which only bear foliage. The former 

 are narrow and lanceolate, while the latter are broad 

 and lobed. The breadth of a leaf would appear to be 

 determined to some extent by the distance beiween the 

 successive leaves, and the ivy affords an example of the 

 rule ; for the internodes of the flowering shoot are generally 

 much shorter than those of the leaf-bearing branches. 

 The upper surface of the leaf is glossy — a provision, in 

 Lubbock's opinion, to prevent snow accumulating on the 

 plant and breaking it down by its weight, as frequently 

 occurs with the branches of trees. Such a provision is, of 

 course, only needed by evergreens like the ivy which do 

 not shed their leaves in the autumn. The duration of the 

 leaves of evergreens varies : on the ivy, holly, and cherry- 

 laurel they remain through the winter, and only fall ofl' 

 when the new ones are developed in the spring ; those of 

 the araucaria, on the other hand, persist for several years. 

 The brown marbling often seen on ivy leaves is due to the 

 presence of a red-coloured sap in some of the cells. 



Fie. V. — Sections of Older Stem. 



Variegation occasionally arises, as in other plants, from the 



absence or defi- 

 ciency in certain 

 parts of chloro- 

 phyll, as the green 

 colouring matter 

 of plants is called. 

 For this vegetable 

 albinoism it is 

 difficult to assign 

 a cause, though 

 we have seen ivy with the dodder growing on it, in which 

 the variegation appeared to be the direct result of the 

 action of the parasite. The ivy appears, however, to be as 

 a rule a remarkably healthy plant, injurious insects and 

 parasitic fungi rarely injuring it. 



The climbing habit of this plant gives it considerable 

 advantage over other species which have to form an erect 

 self-supporting stem. A climber can get its leaves exposed 

 to the light more rapidly and with less expenditure of 

 material than a plant which does not climb. Climbing 

 species are very numerous, especially in tropical forests, 

 where almost every tree may be festooned with rope-like 

 twiners. In such situations the struggle for light is intense ; 

 slow-growing plants, getting overtopped by those of rapid 

 growth, perish, while climbers enjoy every advantage. 

 The latter attain their object in a variety of ways ; the 

 hop and convolvulus by means of a spontaneously revol- 

 ving stem, the passion flower by sensitive curling tendrils, 

 Clematis and Tropieolnm by means of their twisted leaf- 

 stalks, and the bedstrawby the help of its epidermal booklets. 

 In the ivy and creeping fig, the climbing is effected by 

 means of adventitious roots emitted from various parts of 

 the stem and branches. That these are really roots is 

 shown by their deep-seated origin and by the root-collar 

 they form at the point where they burst through the 

 cortical layers of the stem. These roots are, however, 

 merely organs of support, and do not injure the tree on 

 which they grow, as the roots of a parasite like the mistletoe 

 do by absorbing its sap. The ivy is not, therefore, a parasite 

 but simply an occasional epiphyte, which depends on other 

 plants for mechanical support only. Besides climbers, 

 uiany tropical orchids are also epiphytic in their habits. 

 A peculiarity of 



the 



ivy stem 

 greatly 

 it in 



its 



Fig. VI. — Transverse Section of Leaf, 

 tendency being to bend away from 



which 



assists 

 climbing i 

 negative helio- 

 tropism. The 

 stems of most 

 plants bend to- 

 wards the light, 

 but the ivy stem 



is exceptional, its tendency Deing to Deud away 

 light. The effect of this tendency is to press the young 

 branches very close against any supporting wall or tree, so 

 that the little roots are able to adhere firmly, and their 

 outer cells being converted into a kind of mucilaginous 

 cement, the ivy is thus glued to its support. 



On the flower-stalk the surface is slightly furred. The 

 microscope shows this to be due to curious stellate hairs 

 which have an odd resemblance to a starfish. Hairs on 

 the stalks of flowers, as Kerner has taught us, serve to 

 exclude creeping insects, only winged visitors being of use 

 for cross-fertilization. Possibly, these star-shaped hairs 

 serve this purpose in the ivy, but in any case they illustrate 

 the transition from hairs to scales, the latter being merely 

 flattened hairs. 



