198 Practical Plant Biology. 



become tracheids. Here and there rays are seen several cells thick 

 enclosing resin-ducts. Careful examination will also show minute 

 triangular spaces where the wall separating two medullary-ray-cells 

 abuts on that of a tracheid. These spaces represent the cross 

 sections of fine air-passages running inwards along the rays and 

 providing for the aeration of the inner tissues. 



While the segments which are cut off the inner faces of the 

 cambium cells develop in the manner just described into tracheids, 

 the segments similarly separated from the outer faces grow into 

 sieve-tubes. The number of segments formed on the outside is 

 very much fewer, so that the rate of formation of bast is much 

 slower than that of wood. The walls of these outer segments re- 

 main thin and do not become lignified. The nucleus of the cell 

 breaks up into minute fragments and the cytoplasm becomes an 

 exceedingly attenuated layer against the walls. The radial walls 

 develop special areas, sieve plates, which are rounded in outline 

 and are perforated with numbers of very fine pores (say ift or crooi 

 mm. in diameter). Through these pores the protoplasm of ad- 

 jacent sieve-tubes communicates. As the sieve-tube ages these 

 pores are filled up and the communication is cut off. 



The outside of the older parts of the stem is covered with bark 

 which replaces the cortex of the younger parts. The outer parts 

 of the bark are produced by a layer of cells situated in the cortex 

 which retain the power of dividing. This layer is called the 

 cork-cambium. These cells give rise to cells on their outside, the 

 walls of which are altered by the addition of a substance, suberin, 

 to the cellulose. Such walls are very impervious to water and those 

 of adjacent cells easily part asunder. They thus form a covering 

 layer of loose cells which, though waterproof and a bad conductor 

 of heat, allows air to penetrate very slowly. It is called the cork. 

 As the growth of the wood and bast stretches the cortex new cork- 

 cambiums are formed more deeply within the stem, while the 

 outer cork spalls off as rough scales from the surface. 



We have seen that there is direct connection between the wood 

 of the young absorbing roots and that of the transpiring leaves 

 through the outer layers of the wood of root and stem. Even in 

 the lower parts of those stems where the wood is formed of the 

 growth of perhaps many decades of years, by far the greater part 

 of the upward water current is conveyed in a few outer layers. The 

 older more axial wood acts as a skeleton round which the function- 

 ing tissues form a veneer. The carbohydrates and proteins formed 

 in the leaves are usually supposed to pass down the bast to be dis- 

 tributed to the cambium and the growing regions of the branches 



