30 THE BOOK OF NATURE STUDY 



The Pea is a specially suitable plant for detailed study through- 

 out its life-history, and has been rather fully described. The 

 student will be able, however, to supplement the description in 

 many points by his own observations. 



THE IVY (Hedera Helix, L.) 



A number of varieties of the Common Ivy occur in Britain, 

 and many more are in cultivation, since the plant is widely grown 

 as a creeper covering the surfaces of walls and houses. In nature 

 it often grows as a climbing plant covering the trunks of trees or 

 the faces of rocks, but it is also found creeping on the soil of 

 banks, especially in the shade of woods. It is described here, 

 since it illustrates another way in which plants climb, and should 

 be compared in this respect with the Pea and the Honeysuckle. 



When the plant creeps on the 'soil of woods no main root will 

 as a rule be distinguishable. The trailing stem is fastened to the 

 soil by numerous roots which spring from its lower side, and 

 on entering the soil branch freely. In plants climbing up the 

 surface of trees or walls, on the other hand, a well-developed root 

 system will be found. This in plants which have grown from 

 seed comes from the main root, but in those which in cultivation 

 have been produced from cuttings consists of roots springing 

 from the base of the stem. The plant is evergreen and perennial, 

 and the roots continue to extend and their older parts to 

 grow in thickness year after year. Such plants also show a main 

 stem, which branches freely, and this in old plants has often the 

 thickness of a small tree trunk. 



Roots are also produced very freely from the stem. These 

 roots are short and usually unbranched. The old stems are 

 commonly clothed on all sides with a dense covering of them, 

 but on the finer branches they are confined to the shaded side 

 which faces the supporting surface. Here they spring not only 

 from the nodes, but along the length of the internodes. These 

 roots are of use in fastening the plant to the rough surface of 

 the rock or tree-trunk, and may be regarded as special climbing 

 organs. The Ivy, in contrast to the Pea, which climbs by the 

 help of tendrils, and the Honeysuckle, which has a twining stem, 



