366 Transactions. 



Development of Heterophylly in L. volubile and L. scariosum, 



(1.) L. volubile. 



{a.) In no case was heterophylly to be seen in plants under 6 cm. or 8 cm. 

 in length. In such plants the leaves are of the usual acicular form, arranged 

 in an irregular spiral. (&.) In slightly older plants a change becomes ap- 

 parent in the disposition of the leaves on the third or fourth branch, a 

 tendency to dorsi-ventrality being shown in the fact that the two ortho- 

 stichies on each lateral face of the branch are approximating to one by being 

 slightly flattened in the plane of the ground, the leaves, however, still showing 

 the usual acicular form, (c.) Later branches show a still more marked 

 dorsi-ventral arrangement ; there are still seven or eight orthostichies ; on 

 each lateral face of the branch there are two orthostichies approximated 

 to such an extent that they almost have the appearance of being one ; more- 

 over, these laterally borne leaves have developed somewhat extended wing- 

 like bases in the plane of the ground. On the ventral surface of the branch 

 ther'^. are two orthostichies, the leaves of which are of the usual size and form. 

 On the dorsal surface there are also two orthostichies of leaves of the usual 

 form, but slightly reduced in size, {d.) On still older branches the dorsally 

 and ventrally borne leaves are very much reduced in size, and are closely 

 adherent to the stem, and are directed forward towards the apex of the 

 branches (Plate XXXI, fig. 13a), The laterally borne leaves have assumed 

 the marked dorsi-ventrality so characteristic in the mature plant, but 

 the origin of the lateral orthostichy from the approximation of two can 

 still be traced in the fact that its individual leaves point alternately 

 slightly upwards and downwards. There are one or two orthostichies of 

 scale-like leaves on the ventral side, and from one to three on the dorsal 

 side. 



Very often a young plant bearing six to ten branches will show every 

 stage in the change from the spiral to the distichous arrangement and from 

 homophylly to heterophylly. 



(2.) L. scariosum. 



In this species the heterophylly is developed at a very much earher 

 stage than in L. volubile (Plate XXXI, fig. 14), before the young plant has 

 lost its erect habit. At first the leaves are acicular in form, and are borne 

 spirally, although the number of orthostichies is less than in L. volubile, 

 and the leaves are coarser. The change to heterophylly and dorsi-ventrahty 

 is very rapid. The leaves in the two orthostichies of what afterwards becomes 

 the dorsal side of the stem first increase in length, then become very much 

 flattened and extend on either side of the midrib, and finally bend over 

 into a lateral position on either side of the stem. The dorsal origin of these 

 large laterally borne leaves is apparent throughout the life of the plant 

 (Plate XXXI, fig. 15a). The leaves which were originally borne laterally 

 become pushed into a ventral position ; in slightly older plants the 

 ventrally borne leaves gradually become reduced to mere scales. They 

 are borne in regular orthostichies from two to four in number (Plate XXXI, 

 fig. 156). 



From the above account it will be seen that although in L. volubile and 

 L. scariosum there is at first sight a striking similarity in the heterophylly 

 and in the dorsi-ventral arrangement of the leaves, yet the development of 

 this has been different in the two species. 



