TYPES OF STEMS. 375 



thick glossy epidermis over the entire surface. At intervals 

 occur small whitish specks called lenticels, which here perform 

 nearly the same function as do stomates in the leaf. 



730. Bark of trunk. A similar service is performed by the 

 bark for the main trunk and branches of the tree. To admit of 

 growth in diameter the old bark is constantly being thrown off 

 in strips, flakes, etc., and replaced by a new but larger cylinder 

 of young bark. The external appearance thus produced enables 

 experienced persons to recognize many kinds of trees by the 

 trunk alone. 



731. Leaf-scars and bundle-scars. The presence of foliage 

 leaves during the winter would greatly increase the transpiring 

 surface without being of use to the plant; hence they are usually 

 thrown off 'on the approach of winter. The scars left by the 

 fallen leaves are termed leaf-scars. The small dots on the leaf- 

 scars left by the vascular bundles which extended through the 

 petiole into the twig are termed bundle-scars. Sometimes 

 stipule-scars are left on each side of the leaf-scar by the fallen 

 stipules. 



732. Nodes and internodes. The region upon a stem where 

 a leaf is borne is termed a node. The space between two nodes 

 is an internode. 



733. Phyllotaxy. Investigation of a horse-chestnut or willow twig will 

 show that the leaf-scars occupy definite positions which are constant for 

 each plant but different for the two species. The arrangement of the 

 leaves on the stem in any plant is termed phyllotaxy. In the horse- 

 chestnut we find two scars placed at the same node, but on opposite sides 

 of the stem. Somewhat higher up we find two more similarly placed, but 

 in a position perpendicular to that of the first pair. Such phyllotaxy is 

 termed opposite. If in any plant several leaves occur at a node, the phyl- 

 lotaxy is whorled. If but one at each node, as in the willow, the phyllotaxy 

 is alternate. The opposite and alternate types are very commonly met 

 with. Closer observation will show that in the willow, if a line be drawn 

 connecting the successive leaf-scars, it will pass spirally up the twig until 

 at length a scar is reached directly over the one taken as a starting-point. 

 Such spiral arrangement always accompanies alternate phyllotaxy. The 

 section of the spiral thus delineated is termed a cycle. We express the 

 nature of the cycle by the fractions , , f, f, j 6 ^, etc., in which the 



