Holloway. — The Prothallus and Young Plant of Tmesipteris. 



33 



same brown coloration as has the peculiar deposit already referred to. 

 In the vascular cylinder of a medium-grown rhizome, sectioned at some 

 distance behind the apex, there is a tendency for thin-walled elements to 

 invaginate the centrally placed group of xyleni (fig. 82), and in some 

 sections it was seen that it had separated it into two groups. In these 

 rhizomes the brown deposit can be seen in all stages of formation, and 

 it may be detected also in individual cells in the middle cortex, while the 

 fungal coils have almost disappeared from the cortical cells. In fig. 83 

 is shown the vascular cylinder of the largest ground-growing rhizomes of 

 the form T. lanceolata obtained by me in Stewart Island. Here the xylem 

 is definitely split up into two main curving plates more or less surrounding 

 a central group of thin-walled elements. The comparison of a number 

 of sections showed that the configuration of these xylem groups was 

 constantly changing, sometimes two adjacent ends of the groups joining, 

 and at other times one or both of the two main groups subdividing so 

 that the number became three or four. It would seem, then, from a 

 comparative study of the rhizomes of plants of different ages, that along 

 with the increase in number of xylem elements in the central cylinder 

 there is a diminishing disposition on their part to cohere in one group, so 

 that the original monarch condition becomes lost and the xylem is disposed 

 in separate plates or groups in the midst of the phloem, the tendency 

 being in the oldest rhizomes for these groups to be arranged more or less 

 in the form of a ring surrounding a central group of thin-walled (so-called 

 " pith ") elements. It must be noted that this alteration in the xylem- 

 grouping is in no wise occasioned by any branching of the stele. In these 

 very large humus-growing rhizomes also it was seen that the fungal 

 element was almost entirely absent from the cortical cells, nor did the 

 latter show any signs of thickening at their angles. 



The development in size and configuration of the rhizome stele cor- 

 responds in a general way to what Miss Sykes (1908) has described in 

 the gradual differentiation of the stele behind the growing apex of the 

 mature rhizome, except that she refers the splitting-up of the original 

 single xylem group into two or more groups only to the transition region 

 between rhizome and aerial stem. Her material probably did not include 

 such large-sized rhizomes as those examined by me. 



As I have stated above, in the youngest plantlets which show 

 differentiation into both aerial stem and underground rhizome the vascular 

 cylinder is identical in configuration in both. The stele is monarch, the 

 xylem group containing from two to six scalariform elements. In aerial 

 stems of slightly older plants, however, there is a marked change, the 

 characteristic structure of the adult aerial stem, with its separate mesarch 

 xylem strands, beginning to manifest itself. A transverse section of such 

 a young stem shows the pressure of large adherent leaf-bases forming 

 conspicuous angles to the section (fig. 85), the cortical tissue in the angles 

 containing abundant air-spaces. In the central cylinder there are two 

 groups of xylem, obviously mesarch, on the outer side of each of which is 

 phloem, whilst the tissue separating the two groups has the appearance of 

 ordinary parenchymatous cells (fig. 81). I could not identify endodermis 

 or pericycle. There are in young stems of this age no leaf-traces, the 

 leaves as yet being no more than scale leaves. There is, of course, as in 

 all aerial stems, no fungus present. Again, in the aerial stems of still older 

 plants there are to be seen three such separate groups of xylem (figs. 86 

 and 87) placed in the form of a triangle, the position of the xylem 



2— Trans. 



