DIV. I 



MORPHOLOGY 



67 



the membrane toward the water-conducting element and the pit 

 membrane has no torus. On the other side of the wall a simple pit 

 is developed. 



There are transition forms between the various types of vessels, 

 and the thickening bands, in annular and spiral vessels, correspond 

 to the walls of the bordered pits. 



These thickening bars are in fact, as was mentioned above, always narrowed 

 at their attachment to the wall (Fig. 68). As a result of this they are readily 

 detached from the unthickened membrane in the preparation of sections, the spiral 

 thickening often lying within the cavity. The thin portions of the wall between 

 the thickenings correspond to the pit membranes, and, when occurring between 

 two water-conducting elements, 



may be somewhat thickened A. " 



like a torus. 



Annular or spiral vessels 

 are formed in growing parts 

 of plants as they can undergo 

 extension or stretching. 



The thickening of the 

 walls of water-conducting 

 elements increases the 

 mechanical rigidity of the 

 latter and prevents their 

 being crushed bv the ad- r 



. . P .. . J FIG. 71. Tracheides from the wood of the Pine, Pinus 



JOimng living Cells. I he sylvestris. A, Bordered pit in surface view. B, Trans- 

 Hving Contents Of the verse section of bordered pit from a tangential section 



vpwh diminish a<* trip of the wood ' *' torus ' C ' Transverse section of a 

 . Qim tracheide ; m, middle lamella, with gusset, m* ; i, inner 



wall thickens and llltl- peripheral layer, (x 540. After STRASBURGER.) 



mately completely disap- 

 pear. This does not happen in the tracheae until after the transverse 

 walls have been broken through. 



System of Tissue of the Vascular Bundles. The sieve -tubes 

 are usually associated with conducting parenchyma to form strands 

 or bundles of phloem which traverse the plant. The same holds 

 for the tracheides and tracheae, although isolated or grouped 

 tracheides may occur as a water-storage tissue in the parenchyma. 

 Such strands of phloem or of vascular tissue may be regarded as 

 INCOMPLETE VASCULAR BUNDLES. They are common in the secondary 

 permanent tissue as vascular strands in the wood and phloem strands 

 in the bast (cf . pp. 1 54, 1 5 9). In the primary tissues, however, the phloem 

 and vascular strands are united to form COMPLETE VASCULAR BUNDLES 

 which run as a rule parallel to the long axis of an organ, and are 

 united by cross connections into a network. The name VASCULAR 

 BUNDLE SYSTEM is given to this striking feature in the construction 

 of a plant. In these bundles the elements which serve for the con- 

 duction of water are associated with those which conduct organic 



