70 TEXTURE OF VEGETABLES. 



SECTION 7. 



VESSELS. 



1. Simple Tubes. — Fig. 4 a. These are larger than 

 the other varieties, and have no lateral perforations. 

 They occur chiefly in the bark, sometimes in the albur- 

 num or in the fibres of herbaceous plants, but they arc 

 particularly obvious in vegetables which contain thick 

 and resinous juices. The peculiar colour of these jui- 

 ces, which on account of their consistence could not 

 readily pass through a channel of less diameter, will 

 frequently enable us to detect the simple tubes. 



2. Porous Tubes. — Fig. 4 6. These resemble 'the 

 preceding in their general aspect, but the membrane 

 which forms them is not in this case entire ; being 

 pierced with small pores which are arranged in parallel 

 transverse rows. They occur chiefly in woody plants, 

 and are particularly abundant in the hard texture of the 

 Oak, but their uses are not very satisfactorily deter 

 mined. 



3. Spiral Tnbes.. — Fig. 4 c. If a young shoot of the 

 Elder or Snowball is gently broken, its two fragments 

 will appear to be connected by transparent fibres, 

 which when closely examined, even to the naked eye 

 exhibit a spiral form. Fig. 5. 



These are the coats of the spiral tubes uncoiled by 

 the separation of the fragments, and if the latter are 

 still farther separated, these fibres at last are broken, 

 retract and assume their original form. In this state 

 they have been compared to the Trachea or windpipe 

 of animals, but they more closely resemble the elastic 

 wire of suspenders which forms a complete tube ex- 

 cept when forcibly extended and then it represents the 

 spiral vessels in their uncoiled state. These tubes oc- 



