694 SECONDARY GROWTH IN THICKNESS 



Bignoniaceous type, the woody cylinder being continuous, but deeply 

 lobed, or provided with prominent ridges. It is further noteworthy, 

 that in those SAPINDACEAE which are not climbers, the bulk of the 

 family the structure of the stem is perfectly normal, so that the very 

 aberrant construction of the climbing species really seems to be an 

 adaptive feature. 



A third type of anomalous thickening found in liane-stems, is 

 characterised by the appearance of additional complete or partial 

 cambial cylinders external to the primary cambium. Each individual 

 cambial layer produces secondary xylem and phloem in normal fashion 

 for a time ; sooner or later its activity conies to an end, whereupon a 

 new layer of secondary meristem arises further out in the fundamental 

 parenchyma. This process may be repeated several times. Two different 

 sub-types may be distinguished, with regard to the point of origin of these 

 successive cambial layers. The additional meristems may arise either 

 in the primary cortex, as in various Menispermaceae, in Wistaria and 

 Bhynchosia, or in the secondary phloem, as in Gnetum, Bauhinia, 

 Machaeriinn, Phytocrenc, and certain BiC4NONiACEAE (Haplolophium, 

 Glaziovia, Anisostichus, etc.) and Convolvulaceae (Ipomoea, Aryyrcia, 

 Convolvulus). 



A very remarkable type of anomalous thickening, finally, is exem- 

 plified by those lianes in which a secondary cambium arises on the 

 inner side of the normal woody cylinder. In Tecoma radicans, where 

 this phenomenon was first observed by Sanio, the internal cambium 

 gives rise both to xylem and to phloem, but in inverse order. Accord- 

 ing to Scott and Brebner, a similar condition prevails in the stems of 

 Willvfjhbeia firrna (Apocynaceae) and Periploca graeca (Asclepia- 

 daceae). 



The most characteristic anatomical features of liane-stems are 

 those which tend to produce a rope- or cable-like structure, and thus 

 render the organs more pliable. Tensile strength is most marked in 

 the case of the young stem, where the first-formed or central secondary 

 xylem (the " axile wood " of Strasburger) contains a remarkably large 

 proportion of mechanical elements (wood-fibres and fibrous tracheides), 

 whereas its vessels are unusually narrow and its xylem-parenchyma 

 very feebly developed. The later-formed portion of the secondary wood 

 (the " periaxile " wood of Strasburger), on the contrary, contains large 

 vessels and abundant xylem-parenchyma. According to Schenck, this 

 contrast between the central and the peripheral secondary xylem of 

 liane-stems is almost invariably obvious and abrupt. The younger 

 stems are therefore distinctly inextensible in construction, after the 

 manner of roots which have a stereome strand in the centre of the 

 vascular cylinder. As the stems grow older and thicker, however, 



