Foweraker. — Mat-plants and Cushion- plants of Cass River Bed. 9 



Below the rosette the branchlets are clothed with the brown or blackish 

 remains of the leaves which are not closely appressed to the stem. 



A transverse section of a young stem shows the following structure : 

 Epidermis well marked, -with a cuticle of moderate thickness and frequent 

 hairs as described below. Cortex of about six layers of thin-walled poly- 

 gonal cells with few chloroplasts ; middle layers composed of largest cells, 

 innermost have small flat cells. Endoderrnis very regular, composed of 

 barrel-shaped cells considerably larger than the inner cortical ones. Cells 

 often contain anthocyan. 



In older stems secondary thickening and general lignification soon com- 

 mence. Transverse sections just behind the branchlets show the following 

 structure (fig. 1) :■ — Epidermis : In some sections this is still fairly evident 

 with moderately thick cuticle. In others it appears dark, and the cells are 

 flattened and appear to have lost their contents. Cortex : In younger 



remains of 

 epidermis 

 cortex, 

 liquified F^ / ~~*^>>^$^^ ~ e;n ^ od ^ r ^ls 



-phloem, 

 cambium, 



pericycle 

 ■primary xylem/ / "W^CU^f ScUrenx^ym^ 



secondary xylem 



Fig. 1. — Raoulia tenuicaiilis. Transverse section of an old stem. 



parts consists of parenchymatous tissue of polygonal cells with thin cellulose 

 walls. In older parts the whole cortex has become strongly suberized, giving 

 a brownish-yellow colour with chlor-zinc-iodine. Cell-walls are thickened, 

 but have a wavy outline ; the cells have lost their contents, and the walls 

 are commencing to " buckle up." * Endoderrnis : Same appearance as 

 in young stems, save that the cells are larger and have thicker walls. 

 Phloem: Several layers of nearly- equal -sized cells. Xylem and pith: 

 Occupying whole centre of the stem is a strong lignified column of tissue. 

 The pith has become lignified after commencement of secondary growth. 

 Central cells (altered pith) large. Primary xylem masses visible, and zone 

 of small squarish cells more or less arranged in radial rows marks secondary 

 wood. Cambium : Outside wood cylinder cambium visible as a single 

 layer. Pericycle sclerenchyma : At regular intervals near periphery of 

 phloem isolated patches of lignified tissue occur ; these are strands of bast 

 fibres, but more abundant in older stems. 



* In one specimen the cortex became lignified with thick rigid walls, exhibiting 

 clearly the central lamella, as well as pits. The only cells in this specimen which did 

 not exhibit lignification were those forming the two layers just outside the endoderrnis. 

 This cortical lignification, however, must be regarded as unusual. 



