106 



HOLLOW STEM. 



FIG. 54. TWINING OR SPIRAL STEMS : a, 

 French bean ; 6, the hop. 



all flowering plants, however, exhibit some tendency to a spiral 

 growth in their stems. It will be hereafter shown, that the 

 regular arrangement of leaves on the stems and branches, is in a 



spiral line. Moreover, in many 

 smooth-trunked species, as the 

 Cherry-tree, the bark is more 

 easily torn off in a spiral than 

 in any other direction. In trees 

 having few branches, such as the 

 Fir, it is not uncommon to see 

 the same tendency manifested 

 by spiral fissures in the wood, 

 when the bark has been for some 

 time removed. The direction of 

 this kind of twist seems to be as 

 constant in straight stems, as in 

 those which manifest it by coil- 

 ing ; thus, the Common Chesnut, 

 and the Horse-Chesnut, have been observed always to twist in 

 contrary ways. 



148. The stem is not always solid, either in Exogens or 

 Endogens. Thus among the former, the well-known tribe of 

 Umbelliferous plants presents many instances of a hollow stem, 

 as in the common Hemlock ; and in the latter, the Grass tribe 

 affords a corresponding example. In these instances, the hollow- 

 ness of the stem is due to the expansion of the outer portion, 

 faster than the interior can keep pace with it. The young stem 

 is not hollow in either case ; and it is a beautiful instance of 

 mechanical contrivance, that, in these rapidly-growing plants, 

 which are to be rendered independent of support from others, the 

 limited quantity of hard tissue which they form, should be dis- 

 posed at such a distance from the centre, as to give the greatest 

 strength with the least expenditure of material. (See MECHAN. 

 PHILOS. . 83.) If the material of a Wheat-straw, for example, 

 were disposed in a solid form, it would make but a thin wiry 

 stem, which would be snapped with extreme facility. In the 

 hollow-stemmed Endogens. such as Grasses and Bamboos, and 



