HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 117 



latex vessels, circular in cross-section, arranged in rings outside 

 of the cambium and distinguished by their dense contents. In (6) 

 the latex vessels appear as a network, the main parallel longitudinal 

 tubes being connected by horizontal branches. The origin of the 

 vessels can be traced in sections traversing the cambium ; the latex- 

 containing cells are at first separate, but their cavities become 

 continuous owing to fusion of the terminal and lateral walls. 



(2) Latex Cells (Coenocytes). Cut (a) transverse sections of 

 the stem of a Spurge (Euphorbia) and (6) tangential longitudinal 

 sections passing through the cortex. In (a) note the thick-walled 

 latex tubes, lying in the cortex outside the ring of vascular 

 bundles. In (6) note the long tubes running chiefly in the longi- 

 tudinal direction through the cortex, here and there branching but 

 never showing fusion. These coenocytic tubes are formed by the 

 continued growth and branching of single cells which are present in 

 the embryo itself. Note the dumbbell-shaped starch grains em- 

 bedded in the granular contents of the tubes. Carefully cut away 

 the entire cortex from a piece of Spurge stem, boil in a test-tube in 

 potash for a few minutes, and tease out with needles the latex 

 tubes, noting their branching. Cut longitudinal sections of the 

 apex of a Spurge stem, and look for the tips of the tubes, which 

 are rarely seen in the older parts of the stem ; stain with safranin 

 or haematoxylin, and look for the numerous small nuclei at these 

 growing tips. 



140. The Bifacial Leaf. The detailed structure of a 

 bifacial foliage-leaf can be made out by (1) the maceration 

 of entire leaves small entire leaves are most suitable for 

 this purpose ; (2) the removal of the upper and lower 

 epidermis by tearing-off ; (3) examination of tangential 

 sections, cut parallel to the upper and lower surfaces of 

 the leaf ; (4) examination of vertical transverse sections, 

 cut at right angles to the surface of the leaf. 



141. Maceration of Leaf. Boil some Box or Privet 

 leaves for about five minutes in 10 per cent, potash. Hold 

 a leaf under water in a saucer or dissecting-dish, and with 

 scissors cut off a strip of tissue round the margin where 

 the upper and lower epidermis layers are joined. If the 

 leaf has been boiled sufficiently, it will separate readily into 

 three parts : (1) upper epidermis, (2) mesophyll with 

 the veins, (3) lower epidermis. Mount these in water 

 the upper side of the leaf is usually convex and the 

 lower concave, hence the two sides can be distinguished. 



