260 



VEGETABLE. 



Fl s- 



surround the pith of the young shoot, and they form the greater 

 pan of the midrib of the leaves, as well as their reticulations, 

 usually called veins or nerves. They are also contained in 

 the stamens, and pistils, and in the calyx, and cotyledons. 



Reticulated Vessels. These are a modification of the simple 

 spiral above described, and differ from them in having aper- 

 tures, or clefts between the fibres, which give the whole the 

 appearance of net-work, or reticula, and 

 hence the name reticulated. 



This appearance is represented by Fig. 

 229, in which b shows the clefts in the 

 spiral, and , a piece of the same, more 

 highly magnified. 



Reticulated vessels are not found in 

 young plants, being the result of a change 

 produced by its growth. They have been 

 discovered in only a few plants, but in 

 some, as the Balsam (Impatiens balsamina,) 

 at its full growth, they are the only kind 

 of vessels contained in the root ; and in 

 this part they occur most frequently in 

 other plants. 



Annular Vessels. The annular vessels consist of fine mem- 

 branous tubes distended by rings of vegetable fibre, instead of 

 one continuous tube formed by the winding of the fibres, as 

 in the spiral vessels. According to some observers, these 

 vessels are accidental productions, caused by the tearing of 

 the spirals, which are thus made to fail into simple rings, be- 

 tween which a membrane is formed. 



The appearance of this vessel is represented by 

 Fig 230. The rings are generally separated from 

 each other, by a space equal to their own diameters ; 

 but they are sometimes separated to the distance of 

 six, or eight diameters, as seen in the margin of the 

 figure. The rings are retained in their places, by 

 minute needles, which can be separated from their 

 vessels in the Spider- wort, (Trades cantia,) and some 

 other plants. These vessels are too minute to be 

 traced by the naked eye, being here represented 

 several hundred times magnified. 



Punctuated Vessels. These derive their origin both from 

 the simple spiral, arid the annular vessel. The name punctu- 



Fig. 



