82 BOTANY. 



thickening, and as a consequence the perforations, are of 

 various kinds, but generally there is a tendency in the former 

 to the production of spiral bands ; this is more or less evident 

 even when the bands form a network. The transverse parti- 

 tions, which may be horizontal or oblique, are in some cases 

 perforated with small openings, in others they are almost or 

 entirely absorbed. The diameter of the vessels is usually 

 considerably greater than that of the surrounding cells and 

 elements of other tissues, and this alone in many cases may 

 serve to distinguish them. When young they of course con- 

 tain protoplasm, but as they become older this disappears, 

 and they then contain air. 



108. Tracheary tissue is found only in Pteridophytes 



ings, forming the reticulated vessel. After Duchartre. 



and Phanerogams. The principal varieties of vessels found 

 in tracheary tissues are the following : 



(1.) Spiral Vessels, which are usually long, with fusiform 

 extremities ; their walls are thickened in a spiral manner 

 with one or more simple or branched bands or fibres (Fig. 

 72, v", v'", v""}. This form may be regarded as the typical 

 form of the vessels of tracheary tissue. In most cases the 

 direction of the spiral is from right to left.* It is frequent- 

 ly in one direction in the earlier formed spirals and the op- 



* Right to left, in speaking of these spirals, as also in describing the 

 twining of certain climbing plants, is passing up and around in the di- 

 rection of the hands of a watch. Left to right is of course up and 

 around opposite to the hands of a watch. 



