ANOMALOUS SECONDARY THICKENING 080 



production of a massive woody cylinder more readily than any other 

 method of secondary thickening. 



In a considerable number of cases, however, secondary growth in 

 thickness deviates more or less considerably from this normal type. 

 Ontogenetically and morphologically these so-called anomalous forms 

 of thickening are characterised by an unusual disposition of the 

 cambium, and by abnormalities in the structure and arrangement of the 

 secondary tissues. 



From the physiological point of view all the anomalous forms of 

 secondary growth fall under one of two categories. The first of these 

 includes all those cases in which the deviation from the normal type is 

 clearly connected with adaptation to definite external conditions. The 

 bulk of this class is made up of the stems of climbing plants, which are 

 widely different from ordinary vertical woody stems, in respect both of 

 their mechanical requirements and of the conditions under which 

 translocation has to take place. The first group of anomalous organs 

 likewise includes many fleshy roots which serve principally for storage, 

 as well as the submerged stems of certain woody marsh plants 

 (At'schynomcne aspera, A. indica, Herminiera Ela/phroxylon, Pterocarpus 

 Draco, etc.), in which, according to Goebel, the cambium devotes its 

 energies principally to the production of ventilating tissue in the shape 

 of " floating wood." 373 



The second group of anomalous types comprises those instances 

 likewise a considerable number in which the abnormal features are not 

 adaptive but merely represent variations of design. Theoretically, 

 namely, secondary growth in thickness might take place in a variety of 

 different ways ; but the probability of any given type of structure 

 actually appearing in nature largely depends upon its suitability. The 

 determining influence of this consideration is greatest where bulky 

 structures have to be produced. Where, however, a large woody cylinder 

 is not required, modes of secondary thickening which are less efficient 

 than the " normal " form of cambial activity, may suffice. Hence the 

 mechanism of secondary growth exhibits a considerable amount of 

 variation in the case of herbs and small shrubs. The resulting variations 

 of design are all of approximately the same value from the physiological 

 point of view ; in this respect they recall the variations in the cross- 

 sectional outline presented by the stereome (cf. p. 170). 



The extraordinary diversity of the anomalous modes of secondary 

 thickening may now be illustrated by a series of examples, which will 

 be classified in relation to the principles that have just been discussed. 

 It need scarcely be said, that it is quite impossible to give anything 

 like an exhaustive treatment of this subject in the space at our 

 disposal. 



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