THE PBINOIPAL TISSUES. 85 



otherwise they possess the characters of vessels. In one form, 

 as in the so-called wood-cells of Gymnosperms (see paragraph 

 30, page 25) they resemble on the one hand the pitted yes- 

 sels, and on the other the fibres of the wood of Angio- 

 sperms. Every gradation between these tracheides and the 

 other forms of tracheary tissue occur. In another form, as in 

 Cytisus and Celtis, the tracheides are shorter than in the 

 preceding, quite regular in their form, and with tapering 

 extremities (Fig. 75). Their walls are but slightly thickened, 

 and are marked with spirals and pits. When the wall be- 

 tween two contiguous cells breaks through or becomes ab- 

 sorbed the close relation of such tracheides to spiral vessels 

 is readily seen. 



Tracheides may be regarded as composing a less differen- 

 tiated form of tissue, related on the one hand to true tra- 

 oiieary tissue and on the other to fibrous tissue. 



(a) Specimens of spiral vessels with the spirals passing from right 

 to left may be obtained by making longitudinal sections of the stems 

 of Malva rotundifoUa, Impatiens BaUamina, and many other plants. 

 If the thin slices are macerated in nitric acid and potassium chlorate 

 tlie structure may be studied to still better advantage. The spirals in 

 the vessels of Pinus sylvestris pass from left to right ; they may be 

 examined in longitudinal sections of the leaves or young twigs. The 

 stems of Vitis mnifera, Berberis vulgans, Bignonia capreolata, and Ar- 

 temisia dbvotanum furnish examples of vessels, the first formed of 

 which have their spirals running from right to left and the later ones 

 from left to right. Interrupted spirals showing the two directions may 

 be found in stems of Oucurbita. 



(6) Examples of scalariform vessels may be obtained with the greatest 

 ease from the rhizomes of ferns — e.0, of Pteris ; it may also be obtained 

 from many Dicotyledons — e.g., the stems of Vitis. 



ifi) Fine specimens of pitted vessels may be studied in longitudinal 

 sections of many kinds of wood — e.g.,Pirus, Quercus, and Liriodendron ; 

 among herbs, Impatiens and Bicinus furnisli good examples. 



{d) In order to study the tracheides of the Gymnosperms thin slices 

 of the wood — of Pinus, for example — should be heated for some time in 

 nitric acid and potassium chlorate. By this means, after transferring 

 to a glass slide and covering in the usual way, the tracheides may be 

 easily isolated by gently tapping upon the cover-glass, 



(e) Tracheides of the second form are easily studied in horizontal and 

 longitudinal sections of the wood of Celtis. 



