88 SECONDARY GROWTH IN THICKNESS 



Ctordylim, according to Strasburger, become leptocentric (amphivasal) 

 on entering the central cylinder. They are surrounded by typical 

 bast-fibres, or by thick-walled parenchyma. The secondary bundles 

 differ considerably from the primary strands, the functions of 

 mechanical support and water-transport being both allocated to 

 the same organic elements, just as in the woody cylinder of many 

 Gymnosperms and Dicotyledons. The cells in question closely 

 resemble the fibrous tracheides of Conifers, being thick-walled and 

 prosenchymatous, and provided w T ith narrow, oblique (sinistral) bordered 

 pits. In each secondary bundle, the delicate central leptome is sur- 

 rounded by a layer of these tracheides, which is interrupted at certain 

 points, according to Strasburger, by parenchymatous passage-cells 

 (Cordyline). The various secondary bundles are linked together by 

 numerous anastomoses, so as to form a network with vertically 

 elongated meshes. 



The interfascicular secondary parenchyma, which is in a sense 

 comparable to the medullary ray tissue of the Dicotyledonous woody 

 cylinder, displays a distinct radial seriation, which is, of course, dis- 

 turbed in the vicinity of the vascular bundles. The individual cells are 

 furnished with moderately thick and pitted walls. From the physio- 

 logical point of view, this tissue probably corresponds to conducting 

 parenchyma. 



In many species of Dracaena the older roots also undergo secondary 

 growth in thickness. Here the cambial cylinder originates in the 

 pericycle (pericambium) ; so far as is known the secondary tissues are 

 precisely the same as in the stem. 



The results which may be attained by means of the Mono- 

 cotyledonous type of secondary thickening, cannot be more strikingly 

 illustrated than by reference to the historical Dragons' Blood tree 

 {Dracaena Draco), of Orotava (Teneriffe) ; this gigantic tree, first rendered 

 famous by Von Humboldt, was destroyed by a storm in 1869. Over 

 70 feet in height, it had, in Von Humboldt's day, attained a girth, a 

 short distance above the ground, of at least 45 feet. Its age was 

 estimated at approximately 6000 years. 



B. ANOMALOUS FORMS OF SECONDARY GROWTH IN THICKNESS. 



I. GENERAL CONSIDERATIONS. 



The mode of secondary growth in thickness which has been 

 described in the preceding section, may be termed the normal type, so 

 far as Dicotyledons and Gymnosperms are concerned. Its wide 

 distribution may be attributed to the fact that it lends itself to the 



