144 



PHYSIOLOGY 



unaided eye) shows how extensive is the branching (fig. 639). Be- 

 tween these extremes the bundles run, with lateral connections here 

 and there, especially at the nodes, and more or less variation in size 

 and branching. 



Tracheal markings. — The walls of the tracheae are always peculiarly 

 thickened, the thick regions being in the form of rings, or spirals, or a 

 network (figs. 640, 641). The thin parts may be more extensive than the 



thick, as in annular and spi- 

 ral tracheae (figs. 640,0,5; 

 641, s); or they may be 

 mere spots in the midst of 

 the thick wall, as in pitted 

 tracheae (figs. 640, p ; 641, 

 p, r). The thick and thin 

 parts in adjacent tracheae or 

 tracheids correspond; and 

 thus the movement of water 

 laterally, when conditions 

 require it, is facilitated. 



In scalariform tracheids the 

 parts of the wall not thickened 

 are resorbed, and the neighbor- 

 ing cavities communicate freely. 



If water in which some 

 cinnabar has been rubbed 

 up be passed through filter 

 paper, to remove all but the very finest particles, and then the fil- 

 trate is driven under pressure through a piece of fresh pine wood, the 

 pits become choked with cinnabar, showing that water filters through 

 them more easily and so in greater quantity than elsewhere. 



Secondary thickening. — The primary xylem, i.e. that differentiated 

 from the young tissue near the growing points (fig. 642), is adequate to 

 supply only the first leaves. As with age the foliage increases, each 

 primary xylem strand may undergo secondary thickening; i.e. it has 

 added to it similar tissues, originating from a layer of dividing cells 

 which adjoins its outer face (fig. 643). In addition, this meristem (cam- 

 bium), arising between the primary strands, may originate new strands 

 of xylem tissue between the primary ones. These secondary strands 

 may then increase in thickness in the same manner as the primary 



mass s p y c 



Fig. 640. — Longitudinal section (diagrammatic) 

 of a young xylem strand : c, cambium ; y, young 

 trachea, undifferentiated except as to size ; p, pitted 

 trachea; s, s, s, spiral tracheae; a, annular trachea; 

 m, pith. — After Haberlandt. 



