THE STEM. 
61 
more or less fleshy when fresh, as of sanguinaria, 
in other instances they are of the ordinary thickness 
of the overground stem. 
There are some rhizomes that are excessively thick- 
ened, as in the common white potato, and these are 
called tubers . 1 The eyes represent the nodes, from 
which the leaves and branches arise. Tubers should 
not be confounded with tuberous roots, as those of the 
sweet potato and jalap, possessing as they do the mor- 
phological characteristics of roots. 
Instead of the node, or internode, or both, becoming 
excessively thickened, they may be reduced in size and 
crowded upon each other, the leaves at the same time 
becoming thickened and filled with nutriment. Such 
a modified stem and leaves, as in the onion, is called a 
bulb. Bulbs are sometimes produced in the axils of 
the leaves of overground stems, as in some lilies, and 
are then called bulblets. Bulbs can thus be looked 
upon as being in the nature of fleshy buds. 
A corn is intermediate between a true tuber and a 
bulb; it is more in the nature of a thickened inter- 
node, being surrounded in some cases by thin mem- 
branous scales, as in crocus and colchicum. 
Branches are not infrequently modified to hard, 
pointed and spiny structures, as in the Japanese quince, 
when they are spoken of as thorns. Leaves and even 
flowers may arise upon thorns, which shows that they 
are modified branches. 
A number of plants ascend into the air on other 
plants, or other objects which serve as supports, either 
'Swellings on stems or roots may be due to a number of agen- 
cies, viz.: Parasitism or saprophytism of other plants, as mistletoe 
and dodder; the presence of fungi, as on some Coniferse, notably 
pines; the action of insects producing so-called galls on certain 
species of oak, hickory, willow, etc. 
