PLANT ORGANS AND ORGANISMS 137 



Eucalyptus trees of Australia and Tasmania are reported to attain 

 the height of 500 feet. 



Nodes and Internodes. The nodes are the joints of stems. They 

 represent the parts of the stem from which leaves or branches arise. 

 Internodes are the parts of stems between nodes. 



Direction of Stem Growth. Generally the growth of the stem is 

 erect. Very frequently it may be: 



Ascending, or rising obliquely upward. Example: Saw Palmetto. 



Reclining, or at first erect but afterward bending over and trailing 

 upon the ground. Example: Raspberry. 



Procumbent, lying wholly upon the ground. Example : Pipsissewa. 



Decumbent, when the stem trails and the apex curves upward. 

 Examples: Vines of the Cucurbitacea. 



Repent, creeping upon the ground and rooting at the nodes, as the 

 Strawberry. 



Stem Elongation. At the tip of the stem there is found a group of 

 very actively dividing cells (meristem) which is the growing point of 

 the stem. All the tissues of the stem are derived from the cells of 

 the growing point whose activity gives rise in time to three genera- 

 tive regions which are from without, inward: 



1. Dermatogen, forming epidermis; 



2. Periblem, forming the cortex; and 



3. Plerome, forming the fibre-vascular elements and pith. 

 Duration of Stems. 



Annual, the stem of an herb whose life terminates with the season. 

 Examples: Corn. 



Biennial, where the stem dies at the end of the second year. 

 Example: Burdock. 



Perennial, when the stem lives for many years. Example: Oak. 



Stem Modifications. (i) twining, by elongation and marked 

 circumnutation of young internodes as in Convolvulus, Dodder, 

 etc. (2) Tendriliform by thread-like modification and sensitivity 

 to contact of a side branch as in Passion flower, Squash, etc. (3) 

 Spiny, by checking and hardening of a branch that may then become 

 defensive ecologically as in hawthorn, honey locust, etc. (4) 

 Aerial tuberous, in which one or more internodes, enlarge above 

 ground and store reserve food as in pseudobulbs of orchids, Vitis 



