368 Transactions. 



the[ bands in which they are disposed more or less parallel to each other, 

 separated by bands of phloem, the tendency for neighbouring plates to join 

 up with each other being for the most part absent. 



The classification of the six species considered in this study wall then be 

 as follows : (1.) Mixed — L. cernuum, L. laterale. (2.) Banded — («) Radial — 

 L. Billardieri ; (h) parallel — L. volubile, L. scariosum. L. densum. 



It will be borne in mind that in those steles that conform to the parallel 

 type the parallel disposition of the bands will be found only on the dorsal 

 side of the stele, the arrangement on the ventral side being radial on account 

 of the frequent giving-off of the adventitious roots. 



II. The Relation hettveen the " Radial " and the " Parallel " Type. 



In studying the development of the stem-anatomy of L. volubile and 

 L. scariosum it was seen how easily the radial arrangement can pass into 

 the parallel, and vice versa. This is noticeable more particularly in the 

 earlier stages of development, when the protoxylem groups are from five to 

 nine in number. In young plants which have just got beyond the strictly 

 radial stage of triarch and tetrarch, the parallel arrangement ma}^ be at- 

 tained as a mere temporary phase in the course of the various changes in 

 stelar conformation that are continually going on. In still older plants 

 showing from seven to nine protoxylem groups, the parallel arrangement 

 is generally to be found at the point of branching, and is tending to persist 

 also between successive branchings ; it may rapidly pass back into the radial, 

 and vice versa, through the appearance and disappearance (as the case may 

 be) of connections between adjacent bands. In mature plants of L. volubile, 

 L. scariosum, and L. densum the dorsally placed bands are for the most part 

 mutually distinct, and strictly preserve their parallel disposition, although 

 they may be seen now and then to join up with each other. In the larger 

 lateral branches of limited growth of L. volubile and L. scariosum the tend- 

 ency of the bands to join up with each other, and so break the parallel 

 arrangement, is to be observ^ed. In the case of the lateral aerial branches 

 of L. densum the ease with which the radial and parallel arrangements can 

 pass into each other may be seen to an extraordinary degree : branches 

 were examined which showed as many as fifteen to twenty-one protoxylem 

 groups in their lower regions, and the arrangement was here seen passing 

 backwards and forwards from radial to parallel most distinctly several 

 times. 



Now, it may be stated generally that it is in plagiotropic stems that the 

 parallel type occurs, whilst in orthotropic stems the radial type of stelar 

 arrangement is to be found. Jones* points out that the parallel type is well 

 marked in the stem of th^ erect-growing species L. obscurum (Linn.). It is 

 not clear from his account whether or not L. obscurum may be regarded 

 as showing typically the parallel arrangement. At all events, L. clavatum 

 (Linn.), L. annotinum (Linn.), L. complanatum (Linn.), L. chamcscyparissns 

 (A. Br.), and L. alpinum (Linn.), which have plagiotropic growth, are noted 

 by Jones as also showing parallel arrangement, as do also L. densum, L. 

 volubile, and L. scariosum. L. cernuum has plagiotropic growth, but the 

 fact that it shows " mixed " arrangement is considered later. Jones also 

 notices that the radial type is characteristic of epiphytic species of Lijco- 

 podium which show orthotropic growth ; he figures the stele of L. squarrosum 



* Trans. Linn. Soc, March, 1905. 



