SECONDARY THICKENING. NORMAL DICOTYLEDONS. 503 



limit, no accurate comparisons of the different forms of tissue have been carried out, 

 but one may assume from the appearances, that it takes place in an approximately 

 uniform ratio in all. Since then, as was shown above, the different forms of tissue of 

 the same wood usually have a very unequal average thickness of wall, the total relative 

 bulk of the walls at the autumnal limit must depend in the first instance on the dis- 

 tribution of the non-equivalent elements in the annual ring, and secondly on the 

 increase of thickness, which has just been discussed, in each individual form of tissue. 

 Thus, where, for example, the autumnal limit consists chiefly or exclusively of paren- 

 chyma, as in Morus alba, Broussonetia, Fraxinus, Robinia pseudacacia, Caragana 

 arborescens, Amorpha fruticosa, Virgilia, Gleditschia, Catalpa, Paulownia, Ailantus, 

 &c., or of vessel-like, thin-walled tracheides, as in Betula alba, Alnus glutinosa, Populus, 

 Salix spec, and Cytisus Laburnum, while in the inner part of the ring thick-walled 

 fibres prevail, then the autumnal limit is, on the average, thinner-walled than the next 

 inner portion of the ring. The manifold possible combinations and modifications in 

 this respect follow from what has been already stated. 



The Vessels in most Dicotyledonous woods behave similarly to the other elements, 

 in so far as they decrease on the average in width from the spring to the autumn 

 limit of the annual ring, though without, as a rule, undergoing any conspicuous tan- 

 gential flattening. With this a decrease in the number of the vessels from within 

 outwards is often connected. Both differences may appear very gradually, as in the 

 annual ring of the Salicinese, Pomacese, Fagus, Buxus, Cornus sanguinea, &c., and 

 may even be scarcely noticeable (Enckea media, Ulex europseus). In other woods, as 

 Quercus pedunculata, Fraxinus, and Castanea, they appear suddenly, in such a manner 

 that the first spring-zone of the ring, which is rendered highly porous by numerous very 

 Avide vessels, is succeeded on the outside by less numerous and much narrower vessels, 

 occurring between the other elements. According to the degree in which this peculi- 

 arity of the spring-wood, which may be shortly characterised by the name ' porosity,' 

 clearly appears, and coincides with other differences of structure between the annual 

 limits and the middle portion of the ring, the boundary comes out more or less 

 sharply even as seen with the naked eye. The trees last-mentioned form examples 

 of especially sharp demarcation of the annular rings, and this is also the case with 

 Fraxinus, Robinia pseudacacia and Gleditschia triacanthos, as the boundary is marked 

 both by large vessels, and by the other conditions of structure mentioned above. If, 

 on the other hand, the vessels, while of approximately equal width, are uniformly 

 distributed singly or in groups, as in Enckea media, Ulex europaeus, and no doubt 

 Olea europsea also, the limit of the annual rings may be perceptible with difficulty, 

 or scarcely at all, not only when slightly magnified, but even under microscopic 

 investigation. 



These various degrees in the demarcation of the annual rings, at once suggest the 

 conjecture that cases of their entire suppression may occur. This unquestionably takes 

 place as an individual peculiarity. Many plants, e. g. the Araucarise, appear to have 

 a special tendency to this, and such individual phenomena will be discussed below. 

 Plants, in which the demarcation of the annual rings is constantly absent as a specific 

 peculiarity, are certainly rare, and the statements respecting such cases have been so 

 often disputed, that I am unwilling to adduce even those plants in which I was myself 

 unable to find limits between annual zones, — namely the woody Piperaceze, Opuntia, 



