the Parenchymous Part thereof making a new addition to the Insertions within the Wood; and the Lymphæducts a new addition to the Lignous pieces betwixt which the Insertions stand. So that a Ring of Lymphæducts in the Barque this year, will be a Ring of Wood the next; and so another Ring of Lymphæducts, and of Wood, successively, from year to year." Exactly what Grew meant by the term "Lymphæduct" is not always clear. In some cases he seems to refer to the phloem and cambium by this name, and in other cases to the perimedullary zone. The annual rings in Oak, Elm, Ash, etc. came under his observation, and he remarks that the difference between the Spring and Autumn wood, as we should now call it, arises from the fact that "the Aer-Vessels that stand in the inner margin of each annual Ring, are all vastly bigger, than any of those that stand in the outer part of the Ring."
Grew did not enter into the minuter details of histology, except in his description of the spiral tracheids, to which, as we have seen, his attention was first called by Malpighi's observations. He speaks of the spiral as formed of "Two or More round and true Fibres, although standing collaterally together, yet perfectly distinct. Neither are these Single Fibres themselves flat, like a Zone; but of a round forme, like a most fine Thred." He makes the curious statement that the direction of the spiral is constant, being "in the Root, by South, from West to East: but in the Trunk, contrarily, by South, from East to West."
Although it is as an anatomist that Nehemiah Grew is best known, his grasp of external morphology is perhaps even more remarkable. His work on seed structure has already been quoted. He seems to have quite readily detected the true nature of modified stems. He examined for instance the thorns of the Hawthorn, and saw that their structure was axial. In his own words, they "are constituted of all the same substantial Parts whereof the Germen or Bud it self [is], and in a like proportion: which also in their Infancy are set with the resemblances of divers minute Leaves." It should be recalled that Albertus Magnus, the great scholastic philosopher, writing in the thirteenth century, distinguished between thorns and prickles, and noticed
