64 PARTS OF THE STEM. 



nular the tube has the appearance of being composed of rinses. 

 There are also moniliform vessels, resembling a string of beads, which 

 connect larger ones and convey sap from one set of vessels to another. 

 Mosses and lichens have no vascular system, being composed of the 

 cellular tissue altogether. Vascular fibres compose roots and the stem ; 

 and these may readily be split longitudinally, but with great difficulty 

 horizontally, having to cut across the tubes. 



The glands of plants are internal vessels which effect changes in 

 their fluids. They are situated in the cells and on the borders of 

 spiral vessels. The nectaries of flowers, which secrete or make honey, 

 and the small bodies secreting poison at the base of the stings of 

 plants, are external glands. 



The bark of plants, as we have intimated, has two distinct parts, 

 the one internal, woody and cellular, and the other external and cellu- 

 lar. The most internal part is called liber, and the external the corti- 

 cal. These are independent of each other, the 1st being mostly hori- 

 zontal fibres, or medullary rays, and the 2d is perpendicular. In 

 plants acquiring an age beyond a few years, the wood is divided into 

 two parts, the heart-wood and sap-wood, or alburnum. The first, or 

 central part is generally of a brown or dark color and the other, or 

 external part is softer and of a yellowish color. The internal was at 

 first the external part, it having been changed by age from the harden- 

 ing of the matter within its tubes. The external part, or alburnum, 

 being the young wood, its matter has not become solid. 



The stem consists, then, 1st, of wood, the oldest part of which is 

 heart-wood and the newest alburnum, through which the sap ascends ; 

 2d, of bark, through which the sap descends ; 3d, of the pith, the cen- 

 tral portion of the horizontal part, and 4th of the medullary rays, 

 which connect the rind or external part horizontally with the pith and 

 maintain a communication with it. In some plants these are mixed ; 

 and in the annual and herbaceous they are least distinct. The differ- 

 ences appear by comparing the oak, the cabbage and asparagus. 



The fluids of plants. These are 1st, the sap, or ascending fluid, 

 2d, the cambium, or descending fluid, and 3d, the proper juices. The 

 sap is inodorous and limpid, being imbibed through the pores of the 

 roots from the earth in the state of water holding in solution earthy 

 salts and other substances, all of which are converted into sap. This 

 ascends through the woody part to the branches and into the ribs and 

 veins of the leaves, entering all the vessels and cells of the plant. 

 Thus ascending it is always in action, though its energies correspond 

 with the season and age of the plant. It is facilitated by heat, though 

 in very warm weather it is often slow, because of the absence of moist- 

 ure in the soil. The leaves then eagerly absorb it and revive, if sup- 

 plied artificially. The development of buds is dependent on this cir- 

 culation and the storing of nutriment for them on the return of spring. 



