28 BOTANY OF CROP PLANTS 



ternodes by increase in number and size of cells that com- 

 pose internode tissue. 



As a rule, the number of leaves that will be on a twig is 

 already fixed in the bud. Seldom do new leaves originate 

 during the growing season. This point is worthy of special 

 mention: When a twig has made its year's growth, the 

 internodes do not lengthen thereafter during subsequent 

 years. Increase in length of that shoot is due to the addi- 

 tion of other "joints" at the end. The fixed length of 

 old internodes is well proven by the common observation 

 that nails driven into the trunk of a tree, or a small branch, 

 are not elevated above the ground as the tree grows. It will 

 become grown over with wood, but its height above the 

 ground remains the same. A common impression prevails 

 that the branches of a young tree should be started low to 

 the ground, so that they will be at about the right elevation 

 above the ground when the tree reaches maturity. The sup- 

 position here is that the limbs are raised by the growth of 

 the tree. This notion is erroneous. 



Classification of Stems Based upon Their Medium of 

 Growth. — The medium of growth of most stems is air. Such 

 stems may arise from the soil as in nearly all of our ordinary 

 plants, or they may have no attachment with the soil at all, 

 receiving mechanical support from other plants. The latter 

 are called epiphytes. Tillandsia usneoides is probably the 

 best epiphyte among seed plants. It is the . so-called 

 "Spanish moss." Many orchids of the moist tropics are 

 epiphytic. 



The entire shoot system of some plants is underground. 

 This is the case in the ferns. Many plants produce^both 

 aerial and subterranean stems. For example, Canada thistle 

 has horizontal underground stems, and from these are sent 

 up aerial shoots bearing foHage leaves and flowers. Both 



