294 Transactions. 



that there is any constant relation between the leaf-trace system and the 

 number of protox3-lems, the latter in each ultimate branchlet depending 

 n\)on the order of the branch. In the cones both the number of ortho- 

 stiehies of leaves and the number of protoxylem groups is variable : the 

 vascular cylinder may be either pentarch or quadrarch. In the stouter 

 regions of the adventitious roots there are from six to nine groups of 

 x^'lem arranged round the periphery of the central cylinder, which are 

 either joined up with each other or isolated. The protoxylem elements 

 are very few in number. The innermost zone of the cortex is sclerenchy- 

 matous, as in the rhizome. The smallest roots show a single crescentic 

 gi'oup of protoxylem, with a single group of protophloem. Larger roots 

 show two and three groups of xylem, the phloem occupying the centre 

 of the cylinder and extending between each of the xylem groups. The 

 triarch condition is the common one in all medium-sized roots. It was 

 noted that lateral rootlets are frequently borne in pairs, and that they 

 arise by the trichotomous branching of the root-apex and vascular 

 cylinder. A transverse section of a root at such a point shows that 

 the vascular elements of each rootlet are derived from two adjacent 

 groups of protoxylem and the intermediate group of phloem. 



L. volubile. 



Tlie vascular strand in the stems of the youngest prothallial plants 

 of this species examined always showed two small groups of protoxylem 

 separated by a group of protophloem. The inner cells of the cortex from 

 an early stage are slightly sclerenchymatous. The two groups of proto- 

 xylem in the developing stem join across, and the cylinder thereafter 

 passes through a triarch and then a quadrarch radial stage through the 

 splitting of the protoxylem groups. I have previously published an account 

 of the development of the j^arallel arrangement of the plates of vascular 

 tissue in the adult stem both of this species and of L. scariosum (11, 

 pp. 362-64), in which the conclusion is reached that this arrangement 

 is initiated by and persists from the branching of the stem in the plane 

 of the ground. Also, the different mode of the development of hetero- 

 phylly in the lateral branchlets of these two species has there been given. 

 In L. volubile the first indications of heterophylly in a lateral Ijranchlet 

 do not make their appearance until the plant is 4 in. or 5 in. in height 

 (fig. 97). In slightly older plantlets all stages in the development of 

 the heterophylly may be traced in the various branchlets on the same 

 plant (fig. 98). In figs. 99a and 996 is shown a portion of the distichous 

 region of ;ii Ijranch of a mature plant from the dorsal and the ventral 

 sides respectively. The vascular cylinder of the main stem shows a 

 more or less parallel arrangement of the xylem and phloem plates, this 

 being disturbed, however, on the ventral side owing to the giving-oS 

 of the adventitious roots (fig. 92). There are from ten to sixteen proto- 

 xylem groups in the main stems. Each xylem plate is differentiated 

 into vessels and adjacent xylem parenchyma, and each phloem plate into 

 a row of very large sieve tubes bounded on either side by small-celled 

 phloem parenchyma with abundant cell-contents. The whole of the 

 cortex consists of thick-walled sclerenchyma. As in the other species 

 which show the parallel disposition of the vascular tissues — viz.. 

 L. deusum, L. scariosum, and L. fastigiatum — branching of the vascular 

 cylinder always takes place in such a way that the division of the plates 

 of tissue is in a line at right angles to the plane of their arrangement. 

 In the larofe adventitious roots the configuration of the central cylinder 



