308 



CONDUCTING SYSTEM 



represents a single cell, whereas a true vessel arises from a longitudinal 

 series of cells which become secondarily continuous with one another. 

 Even in a fully developed vessel, it is still quite easy to make out the 

 limits of the individual cells or segments of which it is composed. If 

 the transverse septa between the segments are placed at right angles to 

 the long axis of the vessel, the perforation through which communica- 

 tion is established usually takes the form of a large circular hole, 

 encircled by an annular ridge representing the persistent marginal zone 

 of the septum (" simple perforation ") ; where, on the other hand, the 

 septa are more or less oblique, the perforation usually consists of 



several parallel slits (" scalariform 

 perforation "). . 



Tracheides are generally el- 

 ongated, prosenchymatous cells. 

 Their average length rarely ex- 

 ceeds 1 mm. ; in the stems and 

 petioles of Musa and Carina they 

 may be more than 1 cm. in length 

 (with a diam. of "OS-'l mm.), 

 while in the case of Nelumlium 

 sftcciosnm, Caspary has measured 

 tracheides which were over 12 

 cm. long. Vessels, of course, 

 attain to a far greater length 

 than tracheides, but do not on 

 an average exceed 10 cm. (ac- 

 cording to Strasburger). In the 

 wood of Qacrcus pedunculata, 

 tracheae 2 m. in length are 

 not uncommon, while in lloldnia 

 Pscudacacia, a relatively large 

 proportion of the vessels are 1 m. long. Among lianes even longer 

 vessels are not at all unusual. It must be a comparatively isolated 

 occurrence for a single vessel to extend without interruption through 

 the whole extent of the plant-body, or even throughout a single organ. 

 The diameter of the tracheae is also very variable, the widest vessels 

 ("3-"7 mm.) being found among twining plants. 



The walls of vessels and tracheides are always partially thickened. 

 According to the distribution of the thickening layers, it is customary 

 to discriminate between annular, spiral, reticulate, scalariform and pitted 

 vessels or tracheides (Fig. 126); the last-mentioned type includes two 

 varieties, namely, that with ordinary or simple pits, and that in which the 

 pits are bordered. In the spiral type of vessel the number of thickening 



Fig. 120. 



Radial longitudinal section through the hadrome 

 portion of a vascular bundle from the stem of 

 Oenothera odorata. p, conducting parenchyma ; r, 

 annular (protohadrome) vessel; s, spiral vessel, with 

 the coils of its solitary thickening fibre very greatly 

 drawn out ; s { , younger spiral vessel, with two thick- 

 ening fibres which anastomose at certain points ; k, 

 row of wood-parenchyma cells ; n, reticulate vessel ; 

 g, vessel in process of development, with its septa 

 still intact ; c, cambium. 



