590 R. J. HARVEY-GIBSON AND MINNIE BRADLEY ON 



been selected for detailed study and other species are referred to only in so far as 

 they differ from it. 



A transverse section through an internode of P. Rhceas (figs. 1, 2) shows, 

 successively inwards, epidermis, chlorophylliferous cortex, several layers of sclerotic 

 pericycle, vascular bundles arranged in an undulating circle, medullary rays, and 

 parenchymatous medulla. The epidermis has a moderately thick cuticle, and stomata 

 occur in the interfascicular regions. Multicellular hairs are present, as stated by 

 Leger, but a distinct exodermis is absent. The term "exodermis" is applied by 

 Leger to the layer of non-green cells found immediately below the epidermis in 

 many Papaveracese. In P. Rhceas the subepidermal layer contains chlorophyll and 

 its thin-walled cells resemble those of the cortex. The cortex consists of three 

 or four layers of thin-walled parenchyma containing chlorophyll, with prominent 

 intercellular spaces. An endodermis is not distinguishable, although Leger claims 

 to have identified a fairly well-marked one in P. dubium consisting of non-green 

 cells larger than those of the cortex. He draws attention to the danger of con- 

 fusing the endodermis with the pericycle owing to sclerosis. In P. Rhceas sclerosis 

 has advanced so far as to render it impossible to distinguish between endodermis 

 and pericycle, in the adult at least. The pericycle consists of three or four layers 

 of cells polygonal in transverse section and more elongated than the cortical cells. 

 Their walls are strongly lignified and abundantly pitted. In the interfascicular 

 regions this pericycle merges gradually into thin-walled medullary ray parenchyma, 

 while in the fascicular regions it abuts directly on the hard bast. The vascular 

 bundles are arranged in one or more circles according to the region of the stem under 

 examination. The phloem consists of lignified fibres, unlignified sieve tubes, 

 companion cells, and phloem parenchyma. The fibres are narrower and much 

 longer than those of the pericycle. Pits are present on their walls, but these are 

 not so prominent as those of the pericycle. Leger found no pits on the walls of the 

 bast fibres either in P. dubium or the present species. As to the sieve tubes, Leger 

 states that the sieve plates are poorly developed and often unrecognisable, but 

 careful examination, after suitable treatment, shows that the plates are of the normal 

 angiospermic type. The lateral walls are well supplied with sieve fields, broken up 

 into subareas by secondary thickening of the wall as the tube gets older (fig. 3). 

 Later on callus becomes deposited on either side of the membrane. Hill (2) points 

 out that the connecting filaments react differently to reagents after the formation 

 of callus, and regards them as " slime-strings." In P. Rhceas sieve tubes are by no 

 means infrequent, although owing to the small quantity of callus formed they are 

 not always easy to locate (fig. 4). The companion cells are usually as long as the 

 sieve tubes, and in communication with them by groups of protoplasmic threads 

 (fig. 5), as are also the abundant phloem parenchyma, not mentioned by Leger. 

 When callus is formed in these cases the thickening occurs on the side of the sieve 

 tube only. 



