STEMS 



685 



from that just described. In the secondary wood the tracheids and 

 tracheae soon die, though much of the parenchyma remains alive for 

 some years. The living portion of the wood is known as sap-wood or 

 alburnum, while the dead portion is known as heart-wood or duramen. 

 Usually the alburnum and dura- 

 men differ in color and other- 

 wise, owing to the accumulation 

 of excreta in the latter. 



Conductive cells and tissues in non- 

 vascular plants. Thallophytes and 

 liverworts. In vascular plants the 

 first step in the development of con- 

 ductive cells from parenchyma is elon- 

 gation in the direction of maximum 

 conduction. In many lower plants 

 similarly elongated cells more or less 

 grouped into tissues are by no means 

 infrequent (as in the stipes of the larger 

 fungi, fig. 198). Many fungi have elon- 

 gated rootlike organs which conduct 

 foods for long distances. Much the 

 most remarkable conductive system in 

 thallophytes, however, is that in the 

 brown and the red algae, where in the 

 larger species there are central strands 

 of elongated cells with viscous albu- 



FiG. 1012. A diagrammatic cross section 

 of the stem of a dicotyl, the box elder (Acer 

 Negundo), illustrating secondary growth; 

 there are shown three annual growth rings 

 of xylem formed from the cambium; inside 

 of the first ring is pith; the lines traversing 

 the growth rings represent medullary rays 

 and the outer layer represents the bark. 

 From COULTER (Part I). 



minous contents and with transverse 



sieve plates at the widest portions, resembling those found in the leptome of seed 

 plants; there is also a characteristic callus. Tracheids do not occur in the algae, 

 but water plants in general have a better development of sieve tubes than of tra- 

 cheids or tracheae. In liverworts there is but a slight suggestion of conductive 

 tissues, though Porella exhibits elongated cells, and the thallus of Pallavicinia 

 contains a distinct midrib composed of elongated and pitted cells. 



Mosses. In the mosses elongated cells are frequent, especially in the stems, 

 though they occur also in the midribs of many leaves. In the stem of the Poly 

 trichaceae the structural differentiation approaches that found in ferns, the elements 

 having a concentric arrangement; the central region consists largely of thick- walled 

 cells, which, like tracheids, are prosenchymatous dead cells containing air and 

 water (figs. 1013-1016). Often two or more of these central tracheid-like cells are 

 associated in a group with a common thick wall, while the walls between the indi- 

 vidual cells remain thin, suggesting an approach toward lateral fusion (fig. 1014). 

 This central xylem-like tissue is surrounded by a cylinder of living cells with al- 

 buminous contents (fig. 1015). The underground stem or " rhizome " of the Poly- 

 trichaceae possesses a suberized " endodermis," a " pericycle," and a radial triarch 



