Finlatson. — Stem-structure of Leafless Plants. 367 



Stem-structure. — On the outside is a double-layered epi- 

 dermis, with slightly insunk stomata. There is a thick 

 cuticle outside. The cells of the outer epidermal layer are 

 larger than those of the inner layer, and are somewhat 

 different in shape, those of the outer layer being squarish, 

 while those of the inner layer are somewhat elongated in a 

 direction at right angles to the radial one (fig. 7a). The 

 lower epidermal layer is wanting immediately under the 

 stomata. 



Immediately below the epidermis is a ring of chloren- 

 chyma tissue extending right round the stem. This ring is 

 not complete, but is interrupted by the strands of mechanical 

 tissue, which occur in large numbers immediately below the 

 epidermis. So the chlorenchyma and stereom alternate round 

 the stem. Each of the stereom groups is small, and does not 

 extend inwards as far as the vascular bundles. The cells 

 are small, thick- walled, elongated, polygonal in transverse 

 section (fig. la, m. st.). Each group is surrounded (except on 

 the side which is adjacent to the epidermis) by a layer of 

 thin-walled colourless polygonal cells which separate the 

 mechanical tissue from the chlorenchyma. Small groups of 

 stereom cells, surrounded by a layer of thin-walled cells, also 

 occur in the chlorenchyma ; these groups alternate with the 

 subepidermal ones. There are also a few stereom cells on the 

 outside and inside of each vascular bundle, but in this case 

 there is no layer of thin-walled cells accompanying them. 

 The chlorenchyma cells are larger than those of the stereom 

 tissue. They extend inwards as far as the ring of vascular 

 bundles. The stomata occur only above the chlorophyll 

 tissue in the epidermis. The outer chlorophyll - containing 

 cells are elongated in the radial direction, and have the 

 form of palisade tissue ; further inwards they are more ir- 

 regular in shape, with fewer granules and air-spaces between 

 them. 



The vascular bundles are arranged in an oblong ring round 

 the central pith, with medullary rays between them. They 

 are composed of phloem, cambium, and xylem. As the stem 

 grows older, thickening takes place ; the originally flat organ 

 is rounded off, so the assimilatory tissue is not interfered 

 with. A great part of the xylem thickening consists of libri- 

 form cells (fig. 7c, I. t.) ; these are able to support the stem, 

 so the stereom tissue becomes less necessary. 



The cells of the pith are large and polygonal ; those of the 

 medullary rays are elongated radially to a slight extent. 



Anthocyanin is developed in the epidermal tissue of the 

 edges of the shoot, but not of the flattened sides ; probably 

 this may be explained by the fact that owing to the habit 

 of the plant only the edges receive the sun's rays directly. 



