PHANEROGAMIA : AXIAL ORGANS. 



251 



is certainly the case in Pisonia. An old stem of Crassula (?) which I 

 once examined seemed to bear some resemblance to this. Here the 

 wood consisted entirely of wood-cells. Many separate vertical cords of 

 parenchyma were seen scattered through this mass, each having from 

 two to three spiral vessels. 



All the conditions touched upon here still require a minute study of 

 the history of their development. 



164 



C. Stems of Climbing Plants. 



The stems of many tropical climbers (Lianes, Llanos} exhibit a pecu- 

 liar structure, which has long been misunderstood. Even in our own 

 indigenous plants we meet with some indications of it. In the first year, 

 most of them exhibit nothing striking, if we do not regard the generally 

 square stalk as such ; and we find that they have a simple ring of vas- 

 cular bundles, which closes towards the end of the first period of vege- 

 tation into an ordinary wood cylinder. In the following years, however, 

 the peculiarities are more and more 

 strikingly manifested, consisting in the 

 wood not being uniformly developed to- 

 wards the exterior throughout its whole 

 circumference, but ceasing to grow at 

 definite parts, often regularly, and as 

 frequently in a fantastically irregular 

 manner, allowing the substance of the 

 bark to replace it. In this manner stems 

 are produced, which, in a transverse 

 section, exhibit the most varied distri- 

 bution of the wood. We meet with the 

 first indications of this peculiarity in 

 our indigenous species of Clematis 



forming stems (fig. 164.), in the strikingly 

 broad and regularly arranged large me- 

 dullary rays (a) ; and in the six narrower portions of wood (&),. which 

 are not nearly so fully developed towards the exterior as the six broader 

 ones (d). To these we may add the Bigno- 

 niacece. After the wood has continued for 

 some time to be regularly developed, it ceases 

 growing in four different places (figs. 165 

 167. ), so that the bark is no longer pushed 

 outward ; and on the further development of 

 the wood in the remaining places, the bark 

 forms, in the transverse section, four septa 

 of variable thickness between the four portions 

 of wood. 



In some species these cortical masses become 

 a definite degree broader in each succeeding 



165 



161 Clematis vitalba. Transverse section of the stem. , The pith, b, The smaller 

 wood-bundles, c, The large medullary rays, d, The larger wood-bundles, e, The 

 bark. 



165, '66. :;. Transverse sections of stems of the family of the Biynoniacece, natural 

 size. At a we see the wedge-shaped cortical portions entering between the wood. 



165 The four interposing portions (a, 6) may be plainly seen even by the naked eye. 



