160 DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



same in the ivy whose young shoots are plagiotropous with the leaves in 

 two rows, and in this respect differ from those of allied Araliaceae. In 

 the ivy however the form of leaf Q{ the juvenile shoot, which has 3~5-lobed 

 leaves whilst in the orthotropous flowering shoots the leaves are ovate 

 (Fig. 98), cannot at first be brought into connexion with the external 

 relationships ; and the attempts which have recently been made to 

 establish a relationship between the pointed form of the leaves of the 

 juvenile shoot and their combination into a ' leaf-mosaic ' does not help 

 us ; such mosaics are, as unprejudiced observation of nature shows, either 

 inventions, or exceptional cases picked out with the intention of proving 

 the conformity of the form of the leaf with its purpose. The ortho- 



FIG. 98. Hedera Helix, i portion of a branch ending in an inflorescence. The form of leaf upon it is different 

 from that (represented at 2) upon the sterile branch. After Wassidlo. Lehrb. 



tropous radial shoots with a 2 / 5 phyllotax, which is also r> / 8 and 8 / 13 , appear 

 in ivy only after a certain age is reached and are then probably only formed 

 if light of a higher intensity is available than is required for the formation 

 of the plagiotropous shoots. The first leaves which appear in germina- 

 tion are like those of the orthotropous shoots although the seedling has 

 its leaves from the beginning in two rows ; the five-lobed leaves appear 

 only in the second year *. The radial shoots are frequently used in 

 horticulture for cuttings and these may grow up and live for a long time 

 as ' tree-ivy 2 ' ; they occasionally develop at their base shoots which 

 revert to the juvenile form. 



1 Buchenau, in Botan. Zeitung, 1864, p. 236. 



Mistakenly designated var. ' arborea.' 



