LIANE-STEMS 



693 



In the instances that have been considered so far (except for 

 Bauhiwia), the secondary developments always start from a single 

 cambial layer of normal character. In Sapindaceous tendril climbers, 

 on the other hand {Serjania, Paullinia, Thinouia, etc.), several cambial 

 layers are present from the first. These may be arranged in various 

 ways in the cross-section of the stem. Sometimes the central portion 

 of the organ is built up by a principal cambial cylinder, around which are 

 grouped several minor peripheral cylinders ; more rarely there is no 

 central cambium, but only a circle of from five to seven peripheral 

 cylinders, which are all of approximately equal strength. 



Since this second type of secondary thickening was described by 

 Gaudichaud, it has been carefully studied by a number of investigators, 



--S 



Pig. 2SS. 



T.S. through the stem of 

 Serjania sp. (or Paullinia sp.). <, 

 principal or central woody 



cylinder; a, b, peripheral w ly 



cylinders. Nat. size. After 

 Sclileiden. 



Fi<;. 289. 



T.S. through the stem of an- 

 other species of Serjania (or 

 Paullinia). Nat. size. After 

 Schleidefn. 



Fig. 290. 



T.S. through a young inter- 

 node of Serjania caracasana. s, 

 fibrous cylinder ; within, the 

 principal or central vascular 

 cylinder, surrounded by four 

 peripheral cylinders. The prim- 

 ary bundles (leaf-traces) shown 

 in black. 



among whom Nageli and Radlkofer may be specially mentioned. 

 Nageli, who paid special attention to the ontogenetic development of 

 the individual cambial layers, found that the circle of primary leaf- 

 trace bundles is more or less deeply indented at certain points, so that 

 individual groups of bundles tend to be abstricted from the circle even 

 at this early stage. As a matter of fact, the first traces of this peculiar 

 tendency are shown by the procambial strands ; hence, when these 

 become linked up to form a continuous cambial cylinder, the abstricted 

 groups behave as independent circles of strands, and ultimately give 

 rise to so many separate cambial cylinders (Fig. 290). Thenceforth each 

 individual cambial layer behaves in normal fashion, cutting off secondary 

 phloem on the outside and secondary xylem on the inside (Figs. 288, 

 289). The stem thus consists from the first of several separate woody 

 cylinders ; in other words, the cleavage of the wood is not a secondary 

 process (as in the Bignoniaceae), but is predetermined by the behaviour 

 of the apical meristem. It should, however, be stated that the 

 secondary growth of some Sapindaceous climbers approaches the 



