THE SPRUCE FIR 247 



part in conducting water, while the thick-walled 

 autumnal tracheides serve mainly for strength. 

 Ordinary wood-parenchyma . is also absent, the only 

 parenchyma, apart from the rays, being the secreting 

 tissue of the resin canals. On the other hand, the 

 rays themselves are more complex than those of 

 most Dicotyledons. As the wood increases in thick- 

 ness, new secondary rays are started by the cambium, 

 just as we found in the Wallflower. 



The secondary phloem of the Fir is made up of 

 sieve-tubes, variously modified parenchyma, and phloem 

 rays of complex structure. The sieve-tubes are long, 

 pointed elements, resembling in form the tracheides of 

 the wood. They have sieve-plates on their radial 

 walls only (see Fig. 98, ph). The pores of the sieve- 

 plates are excessively minute compared with those in 

 Angiosperms. The sieve-tubes, as in other vascular 

 plants, contain proteids, and no doubt serve to transport 

 them. The phloem parenchyma forms tangential bands 

 between the zones of sieve-tubes. Same of its cells 

 contain starch or crystals of calcium-oxalate, while 

 others become very thick-walled, forming the stone-cells. 



The medullary rays in the phloem consist of two 

 kinds of cells: (1) ordinary parenchyma tous cells 

 which form starch (see Fig. 98^r.s); (2) cells which 

 are more elongated in the vertical direction, contain 

 no starch, and are very rich in proteids (see Fig. 98, 

 a, a). These cells (which are called the albuminous 

 cells) communicate by pits with the sieve-tubes, and 

 are believed to fulfil the same functions as the 

 companion-cells of angiospermous sieve-tubes. 



