312 



CONDUCTING SYSTEM 



the lateral walls are more or less thickened. The walls are almost 

 invariably smooth throughout, that is, devoid of any trace of pitting or 

 other form of sculpture. In the female plants of Milium punctaium 

 and Bryum leucothrix, however, the walls are provided with transversely 

 elongated pits in the distal enlarged portion of the central strand : 

 these pits are particularly numerous in the last-mentioned species. In 



central strands which are mainly 

 thin-walled, the edges of the cells 

 are not infrequently thickened in 

 such a manner, that the tissue 

 presents a somewhat collenchyma- 

 tous appearance in transverse sec- 

 tion (Dicranum scopariwm). In the 

 genus Polytrichum the tracheides 

 of the central strand are arranged 

 in a number of isolated groups, 

 separated from one another by 

 greatly thickened membranes, which 

 are usually reddish-brown in colour 

 (Fig. 130 t) ; within each group the 

 cells are separated from oue another 

 by thin walls. An examination 

 of serial sections shows, that these 

 apparently isolated tracheidal groups 

 are in reality continuous with one 

 another at certain points, and 

 that some of the thin partitions 

 within the groups represent the very oblique septa of the pro- 

 senchymatous tracheides, while the rest consist of the thin regions of 

 longitudinal walls, which become gradually thicker as they are traced 

 upwards or downwards. The thickened tracts of the tracheidal walls 

 thus constitute a stiffening frame-work, which performs the same office 

 as pertains to the various types of thickening exemplified by the 

 tracheides and vessels of Vascular Plants. 



2. The Functions of Vessels and Tracheides. 161 



Botanists formerly believed that vessels and tracheides served for 

 purposes of ventilation ; it is now established beyond any doubt that 

 these structures represent the water- conducting organs of the plant. 

 The modern view is based in the first instance upon observations 

 respecting the nature of the contents of vessels and tracheides at 

 different seasons and at various times of the day. 



Direct inspection of the contents under the microscope was first 



Fig 130. 



Part of a T.S. through the vascular bundle 

 [central strand] of the stem of Pott/trichina juni- 

 perinum ; r, cortical parenchyma ; I, leptome ; ty, 

 central tracheides with thick brown walls ; t. 2 , peri- 

 pheral tracheides with colourless walls. 



