Mar. 1899.] BOTANICAL SOCIETY OF EDINBURGH 191 



of unstable equilibrium results. The disturbance of this 

 equilibrium may come from within or without. Variations 

 in the external conditions induced by artificial cultures, a 

 limited supply of nourishment, partial desiccation, maxi- 

 mum exposure to light, close competition between the 

 filaments compressed into a limited area, would appear to» 

 be sufficient to disturb seriously this equilibrium and to 

 induce to the maximum extent those profound physiological 

 changes involved in the passage from the vegetative to the 

 reproductive state of protoplasm. The change induced in 

 this case is apparently so excessive as to be of a pathological 

 character, inducing a wave of abnormality, as it were, to pass 

 along each filament, the visible evidence of which we have 

 in the numerous curiosities of conjugation. 



On the Histology of some Fossil Woods (Pt. 2). By 

 E. A. Eobertson, M.A., B.Sc. (With Plates.) 



(Read 9th March 1899.) 



The specimens of silicified woods described in this and 

 a former paper form part of the Geological Collection 

 in the St. Andrews University Museum, to the Director of 

 which, Professor W. C. M'Intosh, I am indebted for kind 

 permission granted to examine them. 



Specimen 2, Plate I., is a remarkably pretty piece of 

 stem, measuring about 41 mm. in length. Owing to a not 

 inconsiderable amount of eccentric growth, the section is 

 irregularly ovate, the pith being in the narrower end. 

 The greater diameter is 54 mm., with radii of 39 and 

 15 mm. respectively, the shorter diameter is 31 mm., and 

 its radii 17 and 14 mm. respectively. 



The pith, though small, is distinct, and the earlier rings 

 are quite concentric to it, while the later rings are more 

 or less incomplete, thinning off greatly or altogether on 

 one side, so as to be crescentic in shape, recalling par- 

 ticularly the similar arrangement in a cross section of 

 canker -infected larch stem. Star -shakes, following the 

 lines of the medullary rays, are present. Looking at the 



