34 BOTANY. 



flowering plants. Every gradation between these tracheids 

 and the other forms of tracheary tissue occur. In another 

 form, as in the wood of many common trees and shrubs, 

 the tracheids are shorter than in the preceding, quite 

 regular in their form, and with tapering extremities (Fig. 

 24). Their walls are but slightly thickened, and are 

 marked with spirals and pits. "When the wall between 

 two contiguous cells breaks through or becomes absorbed, 

 the close relation of such tracheids to spiral vessels is 

 readily seen. 



Tracheids may be regarded as composing a less differen- 

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

 cheary tissue and on the other to fibrous tissue. 



Practical Studies. — Here, as in tlie preceding, it is necessary, 

 especially in herbaceous plants, to first determine by a cross-section 

 tlie position of the woody bundles, as tracheary tissue is always con- 

 fined to them. 



{a) Make a thin longitudinal radial section through a bundle of 

 the stem of the Garden Balsam or Touch-me-not (Impatiens). If suc- 

 cessfully made it will show successively, passing outward, ringed, 

 spiral, reticulated, and sometimes scalariform and pitted vessels, 

 with gradations from one to the other, as in Fig. 20. 



(6) Make a thin cross-section of the same and study carefully in 

 connection with the foregoing. 



(c) Make similar sections of the bundles of Indian corn. The 

 large vessels which can be seen with the naked eye in cross-section 

 are pitted. 



(d) Study in like manner the tracheary tissue in the bundles of 

 the pumpkin-stem. Here the large pitted vessels (which are very 

 distinctly visible to the naked eye) have their walls thrown into 

 numerous folds. 



Note.— The large pores which are so distinctly visible in oak, chestnut, 

 hickory, walnut, ash, and many other kinds of woods are pitted vessels 

 like those of Indian corn and pumpkin. 



(e) Excellent scalariform vessels may be obtained from the bundles 

 of the leaf -stalks of ferns, or better still from the underground stem. 

 In the latter the bundles lie adjacent to the thick dark bands of 

 fibrous tissue. 



