SECONDARY THICKENING. NORMAL DICOTYLEDONS. 475 



between which only small medullary rays exist, with an arrangement not correspondino' 

 to that of the primary xylem-plates. The entire secondary growth thus forms a 

 cylindrical strand, without main medullary rays, e. g. Taraxacum, Scorzonera his- 

 panica, Rubia, Thuja, Taxus, Cupressus, &c. 



On the connection of the bundles of secondary wood with the primary plates, 

 see Sect. 152. 



For the further growth of the bundles and medullary rays when once formed, 

 the successive subdivision of the former by new medullary rays, and the structure 

 of the cambial layer, the same general rules hold good as in the stem. 



The divisions in the cambial zone also appear to follow in general the same 

 rules as in the latter, though this remains to be investigated. The frequent appear- 

 ance of secondary intermediate strands m the broad medullary rays, especially of 

 fleshy roots, is worthy of notice. 



As a rule, the beginning of secondary growth takes place in the root imme- 

 diately after the differentiation of the primary tissues ; in the cases mentioned of feeble 

 development on the other hand, as well as in the adventitious roots of Clusia, Cereus, 

 and Piperaceae, it begins relatively late, so that each portion of the root at first 

 remains for some time in the primary condition. 



II. The Wood. 



I. Distribution and form of the zones of secondary thickening. 



Sect. 140. In the native Dicotyledons and Conifers the wood acquires in 

 every period of vegetation an increment of growth, the development of which begins 

 in spring with the unfolding of the buds, and, with the exception of the roots of 

 Dicotyledonous trees, reaches its end in autumn, starting afresh after a period of rest 

 in winter ; in the roots of native Dicotyledonous trees, on the other hand, it continues 

 to make slow progress throughout the winter, and only reaches its end on the 

 beginning of a new period of growth, whereupon it immediately begins again ^. 



The product of each period of secondary growth, corresponding in our climate 

 to an annual period, is, as a rule, distinguished from that of the earlier and later 

 periods, by definite differences of structure in the limiting layers, which are to be 

 described below. It is therefore called an annual zone, annual layer, or annual ring, 

 and its limiting layers just mentioned are called spring-wood and autumn-wood. 



The consideration of the zones of secondary thickening may conveniendy start 

 from those which are severed into distinct annual layers, especially as this is by far the 

 most frequent case. 



The form of the annual rings has been investigated in the case of trees and 

 shrubs. It is well known, and does not require any detailed statement here, that 

 their average breadth shows great variations in the same individual, according to age, 

 and to the action of more or less favourable conditions of vegetation'', and that under 



' Von Mohl, Eotan. Zeitg. 1863, p. 313. 

 ._ ' Compare the works of Nordlinger and R. Hartig, to be cited below.— Farther, H. de Vries, 

 Einduss des Diuckes auf d. Ban, &c. des Holzes, p. 96; Flora, i8?2, p. 241, 1875. 



