THE STEM OR SHOOT. 



129 



The phenomenon of "witches' brooms" is due to 

 the excessive local production of branches on a twig. 

 The Germans call them " Hexenbesen." They occur 

 on many kinds of trees, perhaps as often as any on the 

 common birch, resembling large birds' nests. They 

 are in all cases due to stimulation caused by a subtle 

 attack, at a very early period, on the tissues by a 

 mite (Eviojphyes rudis) in some cases, in others by that 

 of a fungus [Exoascus alni). The wound caused by 

 either mite or fungus induces an excessive flow of 

 nutriment to the affected spot and this expresses itself 

 in hypertrophied branching. 



Tubeuf describes structures resembling " witches' 

 brooms" produced on the Conifer Thtijopsis dolobrata 

 in Japan under the influence of the mycelium of 

 Gseoma deformans; they "consist of leafless non- 

 chlorophyllous axes, dichotomously branched, and 

 with each branch ending in a disc"; the fungal sori 

 are formed under the epidermis of the terminal disc. 

 We see here that the infection induces a reversion to 

 the ancient dichotomous type of branching. 



The large " knaurs " or " burrs " which appear 

 from time to time on elms, oaks, and other trees, in 

 some cases are caused by the lopping off of branches, 

 in others their origin is obscure. 



In the case of an elm which had been grafted in the 

 Jardin des Plantes at Toulouse there was observed 

 to arise below the graft an enormous cushion which 

 produced more than a thousand closely-approximated 

 branches interlaced together. 



At Montpellier a branch of Broussonetia produced 

 within an area of a few centimetres several hundred 

 shoots. The same thing was seen in a mulberry in 

 Toulouse botanic garden. 



From the exposed cut surfaces of stumps of forest- 

 trees, such as the beech or poplar, young regenerative 

 shoots usually arise in numbers from the cambial zone. 



Coulter and Chrysler describe the formation of 

 shoots from the vascular ring after decapitation of the 



vol. i. 9 



