292 Transactions. 



is slightly sclerenchymatous, but the outer cortex right up to the ejj^- 

 dermis is thin-walled and spongy. In between the vascular tissues 

 and the zone of sclerenchyma there is a pericyclic layer about three cells 

 wide. The metaxylem elements are arranged in rows and groups, and 

 the phloem in like manner. The latter is not differentiated into sieve 

 tubes and phoem parenchyma, but is homogeneous. The protoxylem is 

 in broad groups, very much extended around the periphery of the 

 cylinder. In the sterile ultimate branchlets and in the cones the proto- 

 xylem is in three or four groups, which may Ije in connection at the 

 centre or isolated : they are more or less extended peripherally. In 

 the cones the sporophylls are arranged in six orthostichies in alternate 

 whorls of three. There is no constant relation between the leaf-arrange- 

 ment in those parts and the configuration of the vascular tissues. In a 

 previous publication (11, p. 362) I stated that there was such a correspond- 

 ence to be traced, but this statement must now be Avithdrawn, for I have 

 observed that, while the ari-angement of sporophylls does not vary, the 

 vascular cylinder of the cone may be either quadrarch or triarch, accord- 

 ing to the number of branchings that have taken place. A transverse 

 section immediately behind the apex of the main rhizome shows a full- 

 sized central cylinder, at the periphery of which there are tangentially 

 extended protoxylem groups, while the centre of the cylinder consists 

 of phloem and unthickened metaxylem elements. The differentiation of 

 the metaxylem takes place in a regular manner from the protoxylem 

 inwards. A transverse section of the protocormous rhizome of this 

 species on which the young stem-axis and first root have appeared 

 shows that the vascular strand as it passes along the protocorm consists 

 of two broad groups of protoxylem separated by a single elongated 

 group of protophloem. In the developing stem the vascular cylinder 

 increases in complexity through the separation of the two original proto- 

 xylem groups into three and more, the phloem extending between them. 



L. cernuum. 



In the very young plant the vascular strand, which leads from the 

 stem- apex through the protocorm into the first root, consists at first of 

 a single small crescentic group of protoxylem which includes a small 

 group of protoxylem between its two horns. Lower down in this first 

 root the protoxylem separates into two groups and the group of proto- 

 phloem extends between them, Avhile higher up in the young stems the 

 same takes place. A further stage in the young developing stem is 

 reached by the two groups of protoxylem broadening out, and several 

 metaxylem elements being formed between them, and at length joining 

 the two groups together, thus dividing the phloem into two groups. 

 The further growth in complexity of the vascular cylinder takes place 

 by the protoxylem separating into three and more groups, the phloem 

 extending between them. It is to be noted that from the very earliest 

 stage the vascular elements do not preserve constant relative positions, 

 but tend to separate easily from one another and then join together in 

 a somewhat different arrangement. The vascular cylinder of the mature 

 stem of this species has been described by Jones (13, p. 25), and the figure 

 he gives corresponds very closely Avith that given in the present paper 

 (fig. 90). Jones likens the vascular cylinder of L. cernuum to that of 

 Gleichenia, and Boodle (1) describes that of the allied species L. sala- 

 henR.e in the same way. The outer cortex of L. cernuum U sclerenchy- 

 matous, but the main" inner bulk of the cortical tissues consists of thin- 



