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 transitions between the former and leafy branches[13]. There is no reason to suppose, however, that our author was acquainted with the work of Albertus. Grew realised the nature of Bulbs, and points out that "the _Strings_ only, are absolute _Roots_; the _Bulb_, actually containing those _Parts_, which springing up, make the _Leaves_ or _Body_; and is, as it were, a Great _Bud_ under ground."