SOLID PARTS OF VEGETABLES. 



103 



pulpy and fleshy fruits, and the stems of grasses ; consti- 

 tutes the principle part of mushrooms and other cryptogamdus 

 plants. In the bark of plants it is situated under the cuticle, 

 giving its color to the bark, as the same texture imder the 

 human cuticle does to the skin. In the pith of young plants 

 the cells are filled with watery fluids, but in older plants they 

 are empty, or only filled with air. 



118. Vascular or vegetable fiber consists of membraneous 

 tubes tapering to a point at each end; they unite by fre- 

 quent anastomoses^ and thus- form a kind of net-work ; they 

 are protected by a coating of cellular integument ; their sides 

 are thick and almost opake. They extend thi'oughout the 

 whole plant, distributing air and other fluids necessary to 

 vegetation. The vascular system of plants presents a va- 

 riety in respect to form, and the functions wdiich the difier- 

 ent parts perform. Entire vessels exist without any perfora- 

 tion; these convey the proper juices of the plant, and gen- 

 erally contain oils and resinous juices. Porous vessels ex- 

 hibit many perforations ; they often separate and again unite, 

 changing at length into cellular integument. Sjjiral^ or screw- 

 form vessels (Fig. 124, a)^ 

 are also called trachew^ 

 from a supposed analogy 

 to the trachece or br^eath- 

 ing organs of insects; they 

 appear to the naked eye 

 like silvery threads, fine 

 as a spider's web. By 

 pulling asunder carefully 

 the young stems or leaves 

 of some jDlants, we may 

 see these shining fibers 

 still extending from one ^^ 

 severed portion to the 

 other. They are so elastic that they may be somewhat drawn 

 out, and will coil themselves into a bundle w^hen the exter- 

 nal force ceases to act. The fiber turns spirally from right 

 to left. Ducts {pv false trachecB) are vessels in which the inter- 

 nal fiber does not form a complete spiral coil ; they may be 

 annular (Fig. 121, 5), the sides bein^ marked w^ith transverse 

 coils or rings. Monilifonn ducts (from monile, a nocklacc) 

 have a beaded a[)pearance formed by porous cells contracted 

 at their point of union (Fig. 121, c)\ these serve to connect 

 hirgcr vessels, and to convey sap from one set to another. Lac- 

 tferous (or milk) vessels are met with in the inner bark and 



Fig. 124. 



118. Vwcular fibor— Entire vessels— Porous— Spiral— Ducts— Monilifonn— l.nctifcronB, 



