Vegetable Physiology. 149 



."passion Flower, and the exception in that of the Hop. 

 There is, however, in almost all flowering plants, some tenden- 

 v to a spiral growth, and we shall find when treating of 

 leaves that their regular arrangement on the branches is in a 

 spiral line. In many trees the bark, when stripped off, will 

 follow a spiral direction, and spiral fissures are seen in many 

 kinds of wood, after the bark is removed. 



The stems already described have been solid, but both Exo- 

 ens and Endogens often have hollow stems ; as among the 

 former, such as those of the Angelica and Hemlock, and 

 among the latter, those of the grasses. In these cases the 

 hollowness is due to the expansion of the outer portion faster 

 than the interior. The young stem is not hollow, and it is a 

 beautiful instance of mechanical contrivance, that in such 

 rapidly growing plants, which are to become independent of 

 support from others, the limited quantity of hard tissue which 

 they form should be disposed at such a distance from the cen- 

 tre as to give the greatest strength with the least expenditure 

 of material. In hollow stemmed Endogens, as the Bamboo, 

 and Sugar Cane, certain divisions of the stem are seen, which 

 are called nodes, or knots. When the remainder is hollow, the 

 stem is always solid here, and the partition becomes very 

 firm from the interlacing of fibres from all sides ; and when, as 

 in the Sugar Cane, it is filled up with soft tissue, the knot is 

 still solid. The space from one knot to another is termed the 

 internode, and from each of these divisions leaf buds gener- 

 ally spring. 



Some peculiarly modified stems are often confounded with 

 roots. Such are what are usually called bulbous roots, as 

 those of the Onion and Lily, which are in reality underground 

 stems. The base of the bulb is the real division between the 

 stem and the root, and the fibres proceeding downwards from 

 this are the roots themselves. The scales of such a bulb are 

 really but leaves, altered from their usual character, and at 

 the base of each scale is a little bud, placed in just the same 

 relation with the scale as the buds to the leaves on the higher 

 portiens of the stem. In this case, then, the stem is in a con- 

 tracted state, the internodes not being developed, and the 

 leaves and buds of several nodes arising close together. Other 



