362 Transactions. — Botany. 



Stem and Spine Structure. 



The spine closely resembles the stem in internal structure, 

 the only difference being that in the former the pith-cells be- 

 come disorganized, thus leaving a hollow up the centre of the 

 spine, while in the stem they persist as large polygonal cells. 

 The spine ends in a sharp point. Both spine and stem are 

 quite glabrous. 



In transverse section (fig. 3a) the stem is not circular, but 

 rather elliptical. On the outside is the epidermis, with a 

 thick external cuticle interrupted at intervals by the slightly 

 depressed stomata, which are best seen in a longitudinal sec- 

 tion, as the opening of their guard-cells is at right angles to 

 the longitudinal organic axis (figs. 3b, 3c). The ceils of the 

 epidermis are longish in transverse section. Underneath is a 

 single layer of rather large polygonal cells, slightly elongated 

 transversely. These are thin- walled and contain very granu- 

 lar protoplasm, and are interrupted only underneath the 

 stomata (st. cells). 



The epidermis may be considered as 2-layered, this layer 

 being the lower one. Probably its function is to store 

 water. Beneath it lies the chlorenchyma (p. t.), composed of 

 three or four layers of closely packed palisade cells containing 

 chlorophyll granules. These cells are small, polygonal, and 

 elongated in the radial direction of the stem. Just below the 

 palisade tissue are two or three layers of granular, large, poly- 

 gonal cells, similar to those of the subepidermal layer. This 

 tissue together with the chlorenchyma and epidermis occupy 

 one-third of the thickness of the radius of the stem ; the vas- 

 cular tissue, which is immediately interior, occupies almost 

 one-half, and the central pith fills up the remainder. 



Vascular Tissue. 



In the stem of Discaria toumatou no special stereom 

 tissue is present which might serve to keep the stem rigid ; 

 hence this function has to be performed by the vascular 

 tissue, especially by bast fibres and xylem cells. The 

 vascular bundles form a compact ring round the pith ; the 

 medullary rays consist each of a single row of small pro- 

 toplasm-containing cells, elongated in direction of radius of 

 stem and polygonal. 



On the outside of the vascular bundles are groups of very 

 thick-walled prosenchymatous cells without protoplasm. 

 These are the bast fibres ; they are not arranged very re- 

 gularly ; in longitudinal section they are spindle-shaped, 

 tapering at both ends and fitting into each other. In trans- 

 verse section they are ovoid, with a very small opening in 

 the centre (figs. 3b, 3c, b.f.). The rest of the phloem 



