104 PHTSIOLOGICAL VIEWS. 



under side of leaves ; they contain a fluid, at first transparent, 

 but which becomes white, yellow, or of a reddish hue. These 

 vessels are long, branched tubes, wliich anastomose freely, and 

 thus resemble the vessels of the animal system. 



a. The solid substance of plants is all composed of some varieties of cellular and 

 vasciuar tissue. Roots and stems are made up of vascular fibers ; these may easily 

 be split longitudinally, as the vessels in this case are only separated, and the cellu- 

 lar texture easily yields ; but in severing the roots and stems horizontally, greater 

 resistance is to be overcome, since the tubes are to be cut across. 



b. Vegetables, like animals, have a sijstein of cilands, or internal vessels, which are 

 made subservient to the purpose of producing changes in the fluids of the plants ;— 

 thus the sap is converted into the proper juices ; and from the same soil and nour- 

 ishment plants of very different properties are produced. Mirbel, by the aid of the 

 microscope, succeeded in discovering a system of glands in pores or cells, and on the 

 borders of the spiral vessels. There are also external glands, as the nectarifer- 

 ou.g glands in the corollas of flowers and the stings of plants. 



FLUID PARTS OF VEGETABLES. 



119. The difierent elementary fluids in the vegetable body 

 may be considered under three general divisions : the saj?^ or 

 ascending fluid ; the camhiicm, oy descending juice ; the _proper 

 juices. 



The sap is a limpid, inodorous liquid, the elements of which 

 are imbibed from the earth by pores in the radicles of the 

 root. Water, holding in solution various substances, such as 

 earths, salts, animal and vegetable matter, is absorbed by the 

 radicles, which by some unknown process convert this fluid 

 matter into sap ; and then, by means of vessels which form 

 what is called the sap-wood^ or alburnum^ this sap ascends 

 through the stems to the branches ; passing through the woody 

 part of the petioles, and those minute branches of the petiole, 

 which form the ribs and veins of the leaf, it enters into the ves- 

 sels and cells which extend throughout its substance. In the 

 leaves the sap undergoes changes in composition, after which it 

 is returned to the bark. / 



a. The ascend'incf sap is always in circulation, but its energy varies with the sea 

 son and the age of the plant. Heat has an important influence in quickening the 

 ascent of the sap ; yet, during a dry and hot season, it often appears to ascend but 

 slowly. This is because the absorption of fluids from the earth is checked by the 

 dryness of the soil. The plant, by a little stretch of the imagination, may be con- 

 sidered as thirsty, and thus man may seem not only provident, but humane, in ad- 

 ministering to its roots refreshing draughts of water. Even the leaves, at such a 

 period, seem too impatient to wait for supplies by means of the connecting sap- 

 vessels ; for if water be sprinkled upon them, they fail not to use their own power 

 of absorption, and upon sucli an application, may be seen to revive almost instanta- 

 neously. When the moisture of the earth coincides with elevation of temperature, 

 the sap ascends with the greatest rapidity ; this is the case in spring. It is at this 

 period that incisions are made into the wood of maple-trees, in order to procure sap 

 for the manufacture of sugar. The sap may at this time be seen flowing almost in 



a All the solid substance of pl.ints comp-jsed of tissue— ft. Glands. — 119. Tiiree kinds of fluids — 

 What is the sap, and liow formed 1 — What is the use of ti)e sap-wood ? — a. Effect of drought upon 

 the plant — Wliv are incisions made in ma-jle-,rees in the spring, rather than at anv otlier period ? 



