686 SECONDARY GROWTH IN THICKNESS 



balanced by the greatly enhanced mechanical efficiency of the 

 xylem. 



In a few instances (e.g. in certain Willows and in the Canadian 

 Poplar) the heart-wood does not differ appreciably from the sap-wood as 

 regards either its strength or its durability. In such cases, the duramen 

 is liable to decay, and its destruction is often hastened by the attacks 

 of Fungi, so that trees of this type usually become hollow at a com- 

 paratively early stage. 



The gummosis which often affects the wood of the Amygdaleae is 

 a pathological phenomenon. 



III. SECONDARY GROWTH IN THICKNESS IN 

 MONOCOTYLEDONS^' 1 



It has already been pointed out, at the beginning of the present 

 chapter, that, in the great majority of Monocotyledons, the entire 

 growth in thickness takes place in the apical meristem. The stem does 

 not begin to elongate until the growing point has undergone a definite 

 amount of enlargement ; in certain Palms elongation is succeeded by a 

 second phase of enlargement (cf. above, p. 647). But genuine secondary 

 thickening, with addition of entirely new tissues, only occurs among the 

 arborescent Liliifloeae (Aloe, Yucca, Dracaena, Cordyline, Alctris), and 

 in the tubers of the Dioscoreae ; and even in these cases the process 

 differs in certain essential features from the secondary growth in 

 thickness of Gymnosperms and Dicotyledons. 



In the trunks of the aforesaid Liliiflorae, the primary arrange- 

 ment of tissues conforms to the ordinary Monocotyledonous scheme. 

 The course of the bundles is of the same type as in Palm-stems. 

 The cambial layer sometimes originates immediately behind the apex 

 (Yucca aloifolia, Aloe plicatilis, Bcaucarnca tubcrculata), before the 

 primary meristematic layers have become converted into permanent 

 tissues ; in other cases its appearance is deferred until all the per- 

 manent tissues have been differentiated at that level (e.g. in most 

 species of Dracaena). In any case it always arises from one of the 

 innermost layers of the cortical parenchyma that is, on the outside of 

 the vascular cylinder. The elements of this layer undergo activi 

 tangential division, and so give rise to a zone of meristematic cells 

 (Fig. 283 a,?'), which, however, differ in certain respects from genuine 

 cambial elements ; they are, for example, at most two to four times 

 longer than their width, and are not prosenchymatous. Schoute states 

 that at first this cambial layer is regenerated at the expense of the 

 cortical layer immediately external to it ; later on a definite initial 

 layer is differentiated. 





