180 PIIYTOTOMY. 



cannot be unrolled ; (Tab. V. Fig. 3.) They are formed 

 by an original spiral vessel meeting with perpendicular fibres 

 in its sides, which fibres cross the winding lines longitudi- 

 nally, and unite them together. These perpendicular fibres 

 belong to the original structure of the spiral fibres, and are 

 by no means part of the neighbouring cellular texture, be- 

 cause, from the first, we see them as j^eculiar fibres, and 

 not as partitions or membranes ; and because, after a gentle 

 maceration, by which the cellular texture is destroyed, these 

 fibres last as long as the twisted fibres oi' the spiral canals 

 themselves. 



But that this form is not accidental, but one which makes 

 part of the original vegetable structure, is evident from this, 

 that in certain families this appearance is so connnon, that in 

 the Ferns, in the Lycopodea?, and in Grasses, we perceive 

 scarcely any other. In young wood, too, this form appears 

 very early, although, in the first shoots, the primitive spiral 

 vessels, having a great resemblance to the pith, long preserve 

 their unchanged shape. 



283. 



A remarkable variety of the spiral form is that in which it 

 appears porous, punctured, or surrounded by a reticulated 

 covering. This also is an original, and by no means an acci- 

 dental form. It is most frequently observed in the roots, 

 and in the woody parts of plants ; (Tab. III. Fig. 25.) 



The origin of these vessels may be explained in the same 

 way as that of the vasa scalaria ; that is to say, perpendicu- 

 lar fibres cross the winding fibres, and bind them together. 

 To this also is added the further circumstance, that, in the 

 instances we have mentioned, the spiral fibres often cling 

 more early together, and take the shape of bands, which being 

 crossed by the perpendicular fibres, form the net-work above 

 mentioned. Not unfrequently \\e observe particular parts 

 of these canals closely contracted, which gives to them the 

 appearance of bladders. We also sometimes remark oblique 

 fibres, especially in Sassafrass wood, which seem to be re- 

 mains of the original twistings of the threads. To the older 

 tubes, a soft and vesicular cellular texture frequently attaches 



