LECTURE X. 



THE SECONDARY GROWTH IN THICKNESS OF SHOOT-AXES 

 AND ROOTS. 



Having started with the typical three systems of tissues, and traced the 

 anatomical differentiation to its most rudimentary beginnings in the INIosses, Algae, 

 and Fungi, we may now pass to the consideration of the highest stage of development 

 which the formation of tissue undergoes in the vegetable kingdom. We find this 

 chiefly in true woody plants, the trees and shrubs of the natural classes Gymnosperms 

 (especially the Conifers) and Dicotyledons. But in many species of both these 

 sub-divisions, not generally considered as woody plants, exactly the same processes 

 of tissue-formation take place as in perennial woody plants ; in fact much of what 

 is to be said concerning the formation of wood and the processes accompanying 

 it is often especially evident in annual and biennial plants. In anticipation, however, 

 it may be mentioned at once that the phenomena of growth to be treated of here 

 do not, as a rule, occur in water-plants ; since the biological relations connected with 

 subsequent growth in thickness have in general a meaning and purpose only in 

 transpiring land plants. We may comprehend the phenomena here concerned under 

 the idea of the secondary giowth in thickness of shoot-axes and roots'. 



As the whole process of growth in length and the origin of new organs is 

 referred to the growing point of the roots and shoots ; so the entire growth in 

 thickness springs from the functional activity of a thin layer of tissue of similar 

 character — i.e. from the cambium. Where this remains active for a long time, 

 as it does occasionally even for centuries, the originally thin root-fibres and shoot- 

 axes gradually develop into those huge thick bodies met with as old tree-trunks 

 and their branches, and napiform roots and tubers. Although these organs 

 originally (i.e. immediately after the conclusion of their growth in length) are com- 

 posed, as already described, of epidermis, fundamental tissue, and vascular bundles ; 

 later, when the growth in thickness by means of the cambium has continued 

 long enough, but little remains of that original structure, the whole of the thick 

 stem, branch, or root, consisting, apart from insignificant remnants of the primary 

 pith and vascular bundles, entirely of the products of the cambium. With the 



' The extensive subject of the secondary growth in thickness, and especially the numerous 

 abnormal cases not touched upon in the text, are thoroughly and accurately treated in De Har\'s 

 ' Vcrgl. Aiiat. der Vegcfafioiisorgaiic,^ i^77- 



