THOMAS HOPKIRK OF DALBETH. 243 



new roots. He adds that on old fruit-trees such burs 

 occur frequently, and that gardeners take advantage 

 of their readiness to root, by cutting ofP, when the 

 fruit is setting, a branch immediately below one of 

 the burs, and planting it, whereupon the roots shoot 

 out so rapidly that the branch is supplied with 

 nourishment and the fruit perfected. This method 

 of propagation, he says, had begun to be adopted 

 with some success, trees so raised bearing fruit 

 earlier than by grafting, as might be seen at Lance- 

 field at the time when he was writing (September, 

 1816), where there were several young trees loaded 

 with fruit though planted only the preceding season. 

 He shows elsewhere, however, that other causes 

 come into play in the formation of burs, such as 

 too large a supply or improiDer direction of the sap, 

 the bite of insects, or the rupture of vessels. "An 

 undue determination of sap towards one particular 

 branch is," he says, "the cause of the bunches or 

 knots so frequently to be observed in the Birch ; 

 the knot being plentifully su^Dplied with nourishment 

 sends out many shoots and twigs, and forms the 

 bushy appearance called the Witch Knot." Kurt 

 8prengel, a celebrated German botanist of that 

 time, quotes from Hoj)kirk, and states that these 

 Witch Knots consist of buds intermingled in a great 

 variety of ways, from which, however, no proper 

 branches proceed, but only a crowd of thin twigs 

 forming a bush-like mass, and that similar knots 

 were often found in the pines of the very dry 

 sandy plains of Germany. 



The extraordinary exertions made by plants strait- 

 ened for nourishment he illustrates by the case of 

 a Plane - tree (presumably Acer pseuclo-platamis) 

 that grew amongst the ruins of New Abbey, in 

 Galloway, on the top of a wall about 20 feet above 

 the ground. The nourishment in such a situation 

 being insufticient, roots were thrown out till they 

 reached the ground, after which the tree made 

 vigorous progress. Another instance he adduces is 



