THE TISSUES OF THE STEM 65 



at iirst sight confused. To secure conduction, mechanical strength, and 

 storage it is necessary that each of the tissues that meets these needs must 

 be continuous in order to be effective. There is no general or regular rule of 

 their arrangement for all stems. The complex problem is solved by different 

 trunks in different ways. A careful microscopic analysis of the masses of 

 wood and bast is necessary for the understanding of any individual case. 

 But such analysis shows, for instance in the wood, that the three tissue-types 

 form each their own connected system, though all are fused together into the 

 woody column. The clearest example of this is found in the medullary 

 rays, which are intimately related to the parenchyma of wood and bast, and 

 thread together these apparently isolated tracts radially, so as to form a 

 connected living system of storage-cells, which finds its inner limit at the 

 barrier of the dead heart- wood. 



The growing vascular column is covered externally as it expands 

 by the cortex and epidermis. These must necessarily yield in some 

 way to the increase within. Sometimes they simply stretch, and 

 this is usual in most stems for a time. But as a rule the epidermis 

 and some of the cortex dries, and peels off, owing to the formation 

 of cork. The nature and function of a corky tissue is that it forms 

 an impervious barrier to the passage of water. As bottle-cork it is 

 used for this purpose. In the plant a layer of cork, wherever formed, 

 will cut off any tissues outside it from physiological interchange with 

 the tissues within. The first layer may be succeeded by other layers 

 formed more deeply, and cutting off successive bands of deeper 

 tissue. The layers may encroach into the phloem, from which 

 successively the outermost, that is the effete layers, would thus 

 be cut off by an impervious barrier from the active tissues within. 

 All tissues lying outside the innermost cork-layer are called collectively 

 Bark, which is consequently a dead tissue, including it may be 

 epidermis, cortex and phloem. As the stem continues to grow, the 

 bark being dead does not keep pace with it, but splits into fissures, 

 or peels off in scales. Thus the characteristic appearance of the 

 fissured trunks of the Oak or Elm may arise, or the scaly surface of 

 the boles of the Scots Pine or Sycamore. The thick masses of bark 

 in old stems give also mechanical protection, while they form a non- 

 conducting barrier against excessive heat, as witnessed by the survival 

 of Australian Eucalypts after forest fires. 



Cork may originate from the epidermis (Apple, Sorbus), but more 

 commonly from the outer cortex, often from the layer immedi- 

 ately below the epidermis (Elm, Birch, Figs. 43, 44). Divisions appear 

 by walls parallel to the outer surface, and are repeated so that 

 from each parent-cell a row of cells is produced. A certain cell in 

 each row remains narrow and thin-walled, and it continues to grow 



B.B. E 



