EXTERNAL MORPHOLOGY 57 



the Parenchymous Part thereof making a new addition to the 

 Insertions within the Wood ; and the Lytnphceducts a new addition 

 to the Lignous pieces betwixt which the Insertions stand. So 

 that a Ri7ig of Lymphceducts in the Barque this year, will be 

 a Ring of Wood the next ; and so another Ring of Lymphceducts, 

 and of Wood, successively, from year to year." Exactly what 

 Grew meant by the term " Lymphaeduct " 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 A er- 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 observa- 

 tions. 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 them- 

 selves flat, like a Zone ; but of a rojind forme, like a most fine 

 Thredr 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 Westr 



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 pro- 

 portion : 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 



