284 Transactions. 



Below these cells there is a basal cell which is small and has its wall 

 cutinized. 



Adjacent to these water-absorbing cells there are several layers of 

 small, thin-walled cells which do not contain any chlorophyll. 



Small vascular bundles end blindly in the small-celled tissue adjacent 

 to the water-absorbing cells. The vascular bundles are numerous ; both 

 the xylem and the phloem contain parenchyma, and the bundle is surrounded 

 by a sheath of thin-walled, colourless, parenchymatous cells. 



On the upper surface, and especially above the midrib, there are a few 

 excretory hairs which are covered with minute excrescences of calcium 

 oxalate. These hairs are seen in surface view in fig. 9 and in transverse 

 section in fig. 8. 



Stem (fig. 10). — On the outside there is a bark formed of the dead 

 epidermal and cortical cells. The cells of the cortex and of the epidermis 

 have thickened walls, and the epidermal cells have their external walls 

 cuticularized. A few of the epidermal cells are produced into short, stiff 

 hairs with thick walls and a thin cuticle. 



Bounding the bark there is a single layer of large cells with thin 

 suberized walls : this is the endodermis. Inside this there is a pericycle : 

 this is composed of 1-3 layers of small cells with thickened, lignified walls. 



The phloem forms a wide band ; the sieve-tubes are of small diameter ; 

 with the phloem there is a fair amount of parenchyma. 



The xylem forms a wide ring ; it is composed of wood-fibres and a few 

 narrow vessels- which have thickened walls and are very regularly arranged 

 in rows. 



The medullary rays are not very close together ; they are uniseriate, 

 and are composed of small cells with thickened lignified walls. 



The pith is also lignified ; the cells are rounded or polygonal, and have 

 thin walls. 



29. Wahlenbergia albomarginata Hook. 



Habit. — The plant is a small perennial herb, 2-4 in. high, with a 

 branched rootstock j)utting up a few short, erect stems. The leaves are 

 rosulate or crowded on the short stems ; they are |— fin. long, oblanceolate, 

 obtuse, narrowed into a short petiole, entire, thick, and coriaceous ; the 

 margins are white and cartilaginous. The peduncles are leafless, one- 

 flowered, and about 6 in. high. 



Anatomy. 



Leaf (fig. 11). — The upper epidermis consists of very large cells with 

 thickened walls, especially the external ones. In addition there is a thin, 

 rough cuticle. Some of the cells are produced into long, stiff hairs, the 

 walls of which are very slightly cuticularized. 



The lower epidermis is formed of small cells which have their external 

 walls very much thickened. A cuticle is also present. Stomata are con- 

 fined to the lower surface ; the guard-cells are small, and are raised above 

 the other epidermal cells. There are very small guard-cell ridges. 



The chlorenchyma is differentiated into palisade and spongy tissue. 

 The palisade tissue consists of 3 rows of large, thin-walled cells which 

 contain numerous fairly large chloroplasts. The two outer layers are 

 closely packed, but there are small air-spaces between the cells in the 

 third layer. 



