i6o 



PART II. VEGETABLE HISTOLOGY. 



cross-section, and they are so placed together as to splice one 



over the other like the fibres of a rope, forming a hard and strong 



tissue, 411. 



Wood cells exist in several modifications. The common or 



typical form is slender-fusiform, thick-walled, with a continuous 



cavity, and the walls sometimes marked 

 with oblique or other markings, but 

 frequently without them, Fig. 412. 

 Another form, much less common, is 

 distinguished by the possession of trans- 

 verse septa. Fibers of this kind resem- 

 ble the others in shape and in the thick- 

 ness of their walls, and they are fre- 

 quently marked by oblique slits, Fig. 413. 

 (8) Tracheitis, or vasiform cells, are somewhat intermediate 



in their character between wood-cells and ducts. 



Fig. 414 



Fig. 417. 



Fig. 415. Fig. 416. 



Fig. 414. — Transverse section of tracheids from White Pine, a is a 

 bordered pit. Magnified about 250 diameters. 



Fig. 415.— Longitudinal radiai view, showing numerous bordered pits. 

 a, a bordered pit; m, one of a row of medullary ray cells. Magnified about 

 175 diameters. 



Fig. 416. — Longitudinal tangential section, showing medullary-ray cells 

 and tracheids. In this view the pits appear lenticular in shape, and on the 

 margins of the cells, a is one of the pits and m a medullary ray cell. 

 Magnified about 175 diameters. 



Fig. 417. — Forms of wood parenchyma cells. Magnified about 200 

 diameters. 



They differ from wood cells in having their walls less uniformly 

 thickened, which gives rise to pitted, spiral, ring-like or other 



