VII. SCIENCE IN HER CELLS. 141 



both then, and to-day, after twenty years' further time 

 allowed nie, I sun unable to give the least explanation of 

 the mode in which the wood is really added to the in- 

 terior stem. I cannot find, even, whether this is mainly 

 done in springtime, or in the summer and autumn, when 

 the young suckers form on the wood ; but my impres- 

 sion is that though all the several substances are added 

 annually, a little more pith going to the edges of the pith- 

 plates, and a little more bark to the bark, with a great 

 deal more wood to the wood, there is a different or at 

 least successive period for each deposit, the carrying all 

 these elements to their places involving a fineness of basket 

 work or web work in the vessels, which neither microscope 

 nor dissecting tool can disentangle. The result on the 

 whole, however, is practically that we have, outside the 

 wood, always a mysterious 'cambium layer,' and then 

 some distinctions in the bark itself, of which we must 

 take separate notice. 



21. Of Cambium, Dr. Gray's 220th article gives the 

 following account. " It is not a distinct substance, but 

 a layer of delicate new cells full of sap. The inner por- 

 tion of the cambium layer is, therefore, nascent wood, 

 and the outer nascent bark. As the cells of this layer 

 multiply, the greater number lengthen vertically into 

 prosenchyma, or woody tissue, while some are trans- 

 formed into ducts" (wood vessels i) " and others remain- 

 ing && pa/renchyma, continue the medullary rays, or com- 

 mence new ones." Xothing is said here of the part of 



