090 SECONDARY fiROWTTT IN THICKNESS 



II. SECONDARY GROWTH IN THICKNESS OF 

 IIANE-STEMS. 3U 



The mechanical requirements of liane-stems are quite different 

 from those of the trunks and branches of ordinary woody plants, which 

 are almost always constructed with a view to inflexibility. A climbing 

 stem requires above all things to be inextensible. When hanging freely, 

 it has to bear its own weight ; when wound around another living stem 

 [or when attached thereto by tendrils], it is exposed to pulling and 

 shearing strains, in consequence of the expansion of the support by 

 growth, or its agitation by wind. In addition, long liane-stems have 

 to be flexible, in order to escape the risk of their buckling or breaking, 

 when they themselves or their supports are shaken by violent winds. 

 If branches of the supporting tree are broken, 'or if the whole tree falls, 

 the flexibility of the liane-stem is put to a still severer test. Moreover, 

 every climber is itself constantly altering its relation to the support. 

 For as the older grasping organs become useless, the stem gradually 

 slips downwards, and may come to lie to some extent upon the 

 ground ; as a result, it is often thrown into a number of folds or loops, 

 and also frequently becomes more or less extensively twisted. Twining 

 stems are, in addition, often exposed to radial compression, owing to 

 the growth in thickness of the supporting organ. These very special 

 mechanical conditions are responsible for some of the most remarkable 

 of the various anomalous types of secondary thickening ; Fritz Midler 

 was the first to examine such abnormalities from this physiological 

 point of view. 



All long inextensible liane-stems are constructed more or less after 

 the pattern of a twisted rope or cable. This cable-like structure, 

 however, which combines pliancy with a high degree of tensile strength, 

 cannot come into being, if the woody cylinder is developed in the form 

 of a solid mass. The wood must, on the contrary, consist of separate 

 strands, which can slip past one another ; this essential condition can 

 only be fulfilled, when the woody cylinder is split up into more or less 

 completely separated strands, by the interpolation of softer tissues. 

 As a matter of fact, this splitting up of the wood is an anatomical 

 feature common to all liane-stems, although it may be brought about 

 in a great many different ways. Within recent years, Schenck has 

 given a very careful and exhaustive account of climbing stems from 

 this point of view. 



In the less specialised forms of liane-stem, the cambium is normal 

 to begin with, but soon produces xylem and phloem in different 

 proportions at different points of its circumference. This relatively 



