12 INTRODUCTION 



cells, parenchyma, branched sclerenchymatous elements, 

 medullary rays, resin sacs, and horizontal resin ducts. 

 When young these elements are found to be in regular 

 radial rows, but in the older phloem the increased girth 

 rujitures the rows and they become still more irregular 

 through the tangential expansion of the medullary rays. 



(i) The sieve tubes (figs. 6 and 1,8.1.) are small in section, 

 tangential and radial dimensions being 8-20/^ and 5-8 /x 

 respectively. Their length is difficult to determine, but 

 has in certain cases been estimated at 0'8 to 1 mm. The 

 walls are rather thick, with sieve areas of the normal 

 Abietineae type (vide Hill, 1901) ; these sieve areas are 

 especially numerous on the radial walls, where they are 

 miiseriate, and less numerous on the tangential walls, where 

 they are uni- or bi-seriate. In winter the young sieve 

 tubes contain a small amount of protein, but the older ones 

 appear to be empty. 



(ii) The crystal-containing cells (fig. 7, c.c.) are of the 

 same size and shape as the sieve tubes, but have thinner 

 walls. They contain crystals of monohydrated calcium 

 oxalate (CaCo04.H20). These crystals are rhombs of the 

 monocUnic (klinorhombic) system ; the angle of the rhomb 

 is 78-80°, and in polarized light the extinction is straight 

 and the double refraction strong. They are very small and 

 numerous, and occur in two or three series lying in a tan- 

 gential plane. The growth of the crystals often causes 

 radial swellings in the walls. Besides crystals the cells 

 contain an emulsion which responds to stains for protein, 

 tannin, and resin. No nucleus has been observed. 



(iii) The cells of phloem parenchyma are large, 20-40 X 

 20-35/^ in transverse section, but only 60-90/^ in length. 

 They are formed in longitudinal series and have thin 

 cellulose walls. Each cell has a lining layer of protoplasm 

 and a rather large nucleus flattened against one side. Some 

 cells in each series contain tannin (ferric chloride and 

 potassium bichromate tests), whilst intervening elements 

 are entirely without this substance. The osmic acid test 

 also discloses the presence of fat, especially in the older 



