OF FOREST-TREES. 



121 



preyed upon and mined by the most minute and despicable insects, cHAP.Vir 

 besides other casualties and accidents innumerable, according to the ^"■^V^.^ 

 rustic rhime : 



The cair, the wind-shock, and the knot. 

 The canker, scab, scurf, sap, and rot. 



Whatsoever is exitial to men is so to trees ; for the aversion of which they 



System. His researches into the vegetable creation being very minute, I have followed 

 him in the enumeration of the constituent parts of a plant. He justly recommends 

 the Black Hellebore as the most proper subject for dissection. It is a perennial plant 

 of a firm texture, and not too complex, consisting only of a root, radical leaves, and a 

 flower-stem. 



A careful maceration of the parts, a good microscope, and a most delicate touch, are 

 essentially necessary towards investigating the structure of vegetable bodies. Trees, shrubs, 

 and herbs, are organized in the same manner ; but the colour and thickness of their com- 

 ponent parts are different, according to their respective natures. 



The outer bark is the first thing that presents itself to our view. It has the appearance 

 of a fine film, full of irregular meshes, though in reality it consists of two membranes, with 

 a series of vessels between them. These take their course upwards, and as they advance 

 towards the cup of the flower, inosculate with the small vessels of the inner bark, into which 

 they pour part of the juices they have received from the earth and the atmosphere. The 

 fine meshes serve the purposes of inhalent or exhalent pores, according to the circum- 

 stances of the weather. The inner bark is much thicker than the outer. It is made up of 

 several flakes laid evenly upon one another, each of which consists of two membranes in- 

 closing a series pf vessels. These communicate freely through the whole substance of the 

 rind, and as they inosculate with the vessels of the outer bark, so they also communicate 

 with those of the blea. The blea lies immediately under the inner bark. It is one com- 

 plete and single substance, uniform in its structure. It is of a considerable thickness, and 

 is made up of beds of hexagonal cells. In the angles formed by these cells, we observe 

 the vessels of the blea. They pour their contents into the cells, which appear to be reser- 

 voirs for the water imbibed by the plant. Underneath the blea, lies the fourth substance, 

 called the vascular series. Its structure is extremely simple, being a single course of 

 o-reenish vessels lodged between two membranes. It terminates in the nectaria. At a 



S3 



certain season of the year, the juices of the vascular series are very mucilaginous. They 

 are particularly so in the Holly, and seem to be more elaborated than those of the blea. 

 Its vessels have a free communication with the wood and blea. The favourers of a circula- 

 tion assert that, through these vessels, the returning sap descends : but by the most accurate 

 experiments of Dr. Hales, it appears that the vegetable juices do rise and fall in the same 

 series of vessels, and consequently have no circulation. 



The wood, or fleshy part of a plant, comes next to be examined. In this the life of the 

 vegetable seems to be placed. It is universal in the plant, and is made up of strong fibres. 

 From it all the other parts ai"e produced. It shoots a pith inward, and a rind, blea, and vas- 



