’ 
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 
ruptures the rows and they become still more irregular 
through the tangential expansion of the medullary rays. 
(i) The sieve tubes (figs. 6 and 7, s.t.) are small in section, 
tangential and radial dimensions being 8-20» and 5-8 
respectively. Their length is difficult to determine, but 
has in certain cases been estimated at 0°38 to] 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 
uniseriate, 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.) are of the 
same size and shape as the sieve tubes, but have thinner 
walls. They contain crystals of monohydrated calcium 
oxalate (CaC,0,.H,O). These crystals are rhombs of the 
monoclinic (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, 20-40x 
20-35 1 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 
