72 FABRIC OF PLANTS. 



spirally round another, and the latter withdrawn. They 

 all end in a conical point. The spiral vessels exist 

 singly or in bundles in every part of a plant, from the 

 root to the petals, except the bark, constantly accom - 

 panying and ensheathing the straight vessels or fibres. 

 They may be seen in the form of fine gossamer, by 

 gently pulling asunder the stalk of a strawberry leaf or 

 a young shoot of dogwood. 



Malpighi found that they were not separable into 

 rings, but could be drawn out to a great length, and 

 that they are formed every year in the pulp-wood, 

 immediately within the inner bark. 



Grew found that they alternate with the straight 

 vessels, in every part of the wood, and surround and 

 ensheath them in the leaf-stalk, the leaf, the flower, 

 and the fruit. While the straight vessels are formed 

 in spring, the spiral vessels are formed in summer. 



Du Hamel, upon their structure appearing similar 

 to the rings of the windpipe, supposed that they 

 convey air, though he found sap in them during 

 autumn. 



Hill says they contain a limpid fluid at midsummer, 

 and in autumn, but at other seasons are empty. 



Reichel proved by growing plants in coloured fluids, 

 that the spiral vessels exist in every part except the 

 bark and the pith, everywhere conveying nutritive 

 matter, and not air. 



Hedwig in some measure combines the above con- 

 flicting opinions, by describing a spiral vessel as com- 

 posed of a membranous canal for conveying air, and 

 a spiral tube rolled round it for conveying fluids. 



