682 ECOLOGY 



tubes, giving rise to their name, is the presence at their widest parts 

 of perforated oblique or transverse sieve plates, which sometimes occur 

 also on the side walls (figs. 1008, 1009) . The sieve plates may be en- 

 wrapped by a highly refrangible callus, which is easily soluble, disappear- 

 ing when the cell contents are dilute, but reappearing and closing the 

 sieve plate when the contents become less dilute, as in winter. 



In angiosperms sieve tubes are accompanied by companion cells, elements of 

 smaller caliber, whose abundant cytoplasmic contents are connected prominently 

 with the cytoplasm of the sieve tubes (figs. 1007, 1008). Besides specialized 

 elements, conductive areas contain many parenchymatous cells that remain un- 

 differentiated except for elongation; such cells make up the so-called conductive 

 parenchyma, but they do not differ essentially from cortical cells. 



Primary conductive tissues. — The arrangement of conductive ele- 

 ments into strands or bundles. — Sometimes tracheids occur as isolated 

 cells or idioblasts (as in Salicornia, fig. 772), but in such cases they are not 

 to be regarded as conductive cells. Sometimes there are simple strands, 

 such as isolated bundles of sieve tubes and the finer leaf veins, which 

 are composed chiefly of tracheids. But in most cases all the conductive 

 elements are grouped into compound bundles, notably in the ferns and 

 seed plants, although suggestions of conductive bundles as well as of 

 conductive cells are found in many non-vascular plants. A strand 

 made up of conductive elements is known as a vascular bundle or as 

 mestome; mechanical elements usually are closely associated with the 

 conductive elements, the two making up a Jibrovascular bundle. 



Xylem and phloem. — Usually there are two more or less distinct re- 

 gions within the bundle, namely, the xylem which contains tracheae or 

 tracheids, and the phloem which contains sieve tubes and their asso- 

 ciated elements (fig. 760). Sometimes xylem and phloem are indistin- 

 guishable from one another, as in certain hydrophytes. Both phloem 

 and xylem may contain mechanical elements, and just as the conductive 

 elements as a whole are known as the mestome, so the mechanical 

 elements as a whole, either within or without the fibrovascular bundle, 

 are known as the stereom^. The conductive portion of the xylem is 

 known as hadrome (or hydrome), and the conductive portion of the 

 phloem is known as leptome; for example, phloem may contain such 

 stereome elements as bast fibers and such leptome elements as sieve 

 tubes, while secondary xylem may contain such stereome elements as 

 wood fibers and such hadrome elements as tracheae or tracheids. 



