502 SECONDARY CHANGES. 



Deviations from the typical structure occur, in so far as groups of unusually thick- 

 walled tracheides may appear in the different regions of the annular ring. These, as 

 seen in cross-section, form band-shaped, annular segments, of a brownish yellow 

 Colour, similar to that of the autumn wood ; in Pinus sylvestris they always occur in 

 the innermost annual rings, and often also in the outer ones. Comp. Sanio, Pringsh. 

 Jahrb. IX. loi. 



In the Dicotyledonous woods the demarcation of the annual ring is further influ- 

 enced by the distribution of non-equivalent forms of tissue, and of the changes which 

 each of these shows in the successive zones of the annual ring. 



The shortening of the radial diameter at the autumnal limit, sometimes occurring 

 suddenly and sometimes gradually, and either amounting to a decided flattening or 

 only occurring in a slight degree, according to the special case, is here also general : 

 leaving out of consideration the vessels, which will be specially mentioned below, it is 

 approximately uniform in the forms of tissue which occur in contiguity, more rarely 

 it takes place to a very xmequal extent : as in the marked and sudden flattening of 

 the tracheides of the autumn-wood, while the radial shortening of the fibrous cells which 

 accompany them is gradual and relatively much slighter, in Clematis Vitalba, and 

 Mahonia Aquifolium. 



With reference to the increase of the absolute thickness of the wall at the 

 autumnal limit, a distinction must be drawn between the particular forms of tissue, 

 for the latter differ from one another in average thickness in all the parts of any 

 wood. In the case of equivalent forms of tissue both conditions occur, according to 

 the kind of wood, namely, either a marked increase of the thickness of wall at the 

 autumnal limit or its approximate constancy throughout. The latter, for example, 

 is the case — 



In the bundle-parenchyma of Gleditschia triacanthos, Ailantus glandulosa, 

 Sophora japonica, and Caragana arborescens ; 



In the fibrous cells of Berberis, and Mahonia ; 



In the vessel-like tracheides of Betula, Alnus, Populus, Salix spec, Magnolia 

 acuminata, Sambucus nigra, &c. ; 



In the thick-walled fibriform tracheides of Cornus sanguinea, Syringa vulgaris, 

 and Buxus sempervirens. 



Increase of the thickening of the wall at the autumnal limit takes place for 

 example in — 



The bundle-parenchyma of Gymnocladus, Morus alba, Broussonetia, Paulownia, 

 and Amorpha fruticosa ; 



In the woody fibres of Laurus Camphora, Jatropha Manihot, and Carpinus Betulus; 



In the vessel-like tracheides of Caragana arborescens, Carpinus Betulus, and 

 Ostrya virginica ; 



In the fibriform ones of Syringa Josiksea, Philadelphus coronarius, Kerria 

 japonica, &c. 



Contrary to the rule holding good for other woods, Sanio observed a decrease 

 of the thickness of the wall at the autumnal limit, in the tracheides of Staphylea pin- 

 nata, both as compared with the inner ones of the same annual ring, and with those 

 of the spring-wood of the next following ring. 



As regards the degree of increase in the thickness of the walls at the autumnal 



