242 Phytotomy founded [Book il 



in the form of a tube, and this would answer to an air-vessel 

 in the plant. We should notice specially that Grew, better 

 taught than the phytotomists of the 18th century, considers 

 the vessels of the wood as air-passages, though they some- 

 times convey water. But he goes on with his description 

 of the wall of the vessel ; the flat surface disclosed by the 

 unwinding of a vessel is, he says, itself composed of many 

 parallel threads, as in an artificial ribbon, and the threads that 

 are spirally wound answer to the warp in an artificial tissue, 

 being held together by transverse threads, which correspond 

 to the woof. To realise to ourselves this very strange idea of 

 the structure of a spiral vessel as it appeared to Grew, we 

 ought to know that he thinks that all cell-walls, even those of 

 the parenchyma, are composed of an extremely fine web ; his 

 previous comparison of cell-tissue with foam was only intended 

 to make the more obvious circumstances clear to the reader ; 

 his real idea is, that the substance of the walls of vessels and 

 cells consists of an artificial web of the finest threads. He 

 hints at this on pages 76 and 77, and on page 120 he 

 returns once more to this conception and dwells upon it 

 at great length. The most exact comparison, he says, which 

 we can make of the whole body of a plant is with a piece 

 of fine lace-tissue, such as women make upon a cushion ; 

 for the pith, the medullary rays, and the parenchyma of the 

 rind are an extremely delicate and perfect tissue of thread. 

 The threads of the pith run horizontally like the threads in a 

 piece of woven stuff, and form the boundaries of the numerous 

 vesicles of the pith and the rind, as the threads in a web bound 

 the interstices in it. But the woody fibres and the air-vessels 

 are perpendicular to this tissue, and therefore at right angles to 

 the horizontal threads of the parenchyma, just as the needles 

 in a piece of lace work that lies on the cushion are per- 

 pendicular to the threads. To complete the comparison we 

 ought to suppose the needles to be hollow and the tissue of 

 thread-lace in a thousand layers one above another. Grew 



