364 CONDUCTING SYSTEM 



are typically inflexible in construction, a leptonie which is external, 

 can readily become associated with protective fibrous strands, or with a 

 continuous mechanical cylinder. In special circumstances, on the other 

 hand, this necessity of protecting the leptome may lead to an inverted 

 orientation of the vascular bundles. In the genus Ccntaurca, for 

 example, the small [accessory] cortical strands often have their leptome 

 portions directed towards the subcortical stereome, while the hadrome 

 portions face the periphery ; here the inversion results in a further 

 advantage, inasmuch as the water-conducting hadrome is thereby 

 brought into closer relation with the cortical photosynthetic system. 

 Similar deviations from the normal arrangement of the collateral bundle 

 are not at all uncommon. 



We have still to consider two types of vascular bundle, which may 

 both be readily derived from the simple collateral strand. It has 

 already been stated, that the bicollateral bundle is only distinguished 

 from the simple collateral type by the presence of an additional leptome 

 group on the inner side of the hadrome. The duplication of the 

 leptome should probably be attributed to an increase in the physiolo- 

 gical demands made upon this section of the conducting system, a view 

 which is supported by the fact that steins provided with bicollateral 

 bundles frequently contain isolated accessory leptome-strands as well ; 

 this latter condition is exemplified by the Cucctkbitaceae and Cichori- 

 aceae, and by many Solanaceae. It remains to be explained, why a 

 plant should prefer the addition of an inner strand of phloem to a mere 

 enlargement of the existing outer strand. Once more the decisive 

 factor appears to have been the necessity for ensuring effective pro- 

 tection of the vulnerable leptome-tissue. In the chapter dealing with 

 secondary thickening (Ch. XIV.), a full description is given of the 

 special arrangements for local protection in the shape of intrusive 

 strands and plates of fibrous tissue which often become necessary 

 where the peripheral leptome is bulky. In axial organs of annual 

 duration, the same effect can be obtained more economically by shifting 

 a portion of the leptome tissue to the inner side of the comparatively 

 resistant hadrome strands, where, quite apart from the local protection 

 afforded by the latter, the tensions due to curvature are, in any case, 

 mitigated on account of the greater proximity to the neutral axis. 



It is possible, however, that the duplication of the leptome-strands 

 serves to provide separate conducting channels for two opposite currents 

 of plastic material. A. Fischer states, that the dorsal leptome-strands 

 in the bicollateral foliar bundles of Gucurbita, which are specially 

 active in the young leaf, are empty when that organ is full- 

 grown, whereas the sieve-tubes of the ventral strands always retain 

 their contents even in adult leaves. From these facts Strasburger 



