154 Dr. H. Franklin Parsons on tlie 



species of Riccia among the liverworts, and certain exotic Mar- 

 sileacese (an order allied to the ferns), viz., Salvinia and Azolla, 

 which may be seen in the Victoria Regia House at Kew. 



5. Plants with long slender twining herbaceous stems and 

 heart-shaped leaves are found in a number of different orders. 

 As examples among plants native or cultivated in Britain, we have 

 the two common native species of Convolvulus and several culti- 

 vated ConvolvulacesB, such as the C. (Ipomcea) major; also Poly- 

 (jonum Convolvulus, and the black bryony (Tamus communis), the 

 sole British representative of the order of the yams (Diosco- 

 reacese), an order in which, like the Couvolvulacese, this habit is 

 frequent. I might also add the French bean, in which the 

 leaflets of the pinnate leaf are heart-shaped : and the hop, in 

 which, though the fully developed leaves are lobed, the early 

 leaves and those on the flowering branches are heart-shaped. 



The climbing habit, a development, as Darwin has shown, of 

 the movements of nutation common in growing shoots, enables 

 weak-stemmed plants to get the advantage of more light and air 

 than they would otherwise be able to obtain, by availing them- 

 selves of the support of surrounding objects ; and this habit, it 

 will be seen, has been acquired by plants belonging to a number 

 of different orders. The heart-shaped leaf is probably the form 

 which, under the circumstances, exposes the largest surface to 

 the light. 



6. Plants which climb otherwise than by twining have com- 

 monly palmately-lobed or divided leaves with long stalks, the 

 stalk being at an angle to the plane of the blade. Familiar ex- 

 amples are the ivy, the vine, the Virginia creeper [Ampelopsis), 

 the passion-flower, and the gourds (Cucurbitaceae), including the 

 white bryony, our solitary British representative of the order. 

 The hop also, a twining plant, has leaves of a similar shape 

 during its period of most vigorous growth. 



The advantage of this form of leaf to plants clinging to a 

 support seems to be that the long petiole carries the leaves well 

 out into the air, while the broad spreading blade of the leaf is 

 inclined at an angle which allows it to get full exposure to the 

 light. It is to be remarked that the bushy flowering branches 

 of the ivy, which do not cling to a support but stand out free in 

 the air, have simple ovate leaves with short stalks, the form of 

 leaf of the climbing shoots not being necessary or advantageous 

 on the branches exposed on all sides to the air. 



Other climbing plants have pinnate leaves, as the vetches and 

 other Leguminiferae ; the climbing inmitovy {CorydaHs claviculata), 

 which resembles a vetch in its habit of growth ; many species 

 of Clematis, and the garden plant Eccremocarpiis scaher (nat. 

 order Bignoniaceae), the fohage and stem of which closely 

 resemble those of a clematis, though the flowers are entirely 

 different. 



