120 



BOTANY 



Climbing Stems. Field Work. — A field excursion may be made for 

 the purpose of obtaining as many kinds of climbing stems as possible. Place 

 them in one of the following classes: — 



(1) Stems which twist or twine. 



(2) Stems having roots as holdfasts. 



(3) Stems having parts of leaves or branches as holdfasts. 



Stems may twist around an object in order to climb. Such a plant is 

 the morning glory. Here the stimulus which draws the plant upward is 



evidently the sun. In stems which make use of this 

 method of climbing, it is noticed that each stem twines 

 around the support in a given direction, some revolving 

 with the course of the sun, others in the opposite direc- 

 tion. When such a stem touches an object during its first 

 growth, it is immediately stimulated to turn toward the 

 object and coil around it. 



Leaves and Stems modified as Holdfasts.' — In the 

 common nasturtium (TropoBolum) the leaves revolve in 

 much the same manner as do the stems mentioned above. 

 This movement results in some of the leafstalks fasten- 

 ing around supports, thus drawing the stem up. 



Tendrils. — In some plants definite climbing organs, 

 known as tendrils, are developed. A tendril, which has 

 the appearance of a much-twisted stem, may be modi- 

 fied from part of a leaf, as an entire leaf or as part of a 

 branch. Tendrils have the habit of at first stretching 

 out as far from the main stem as possible, then slowly 

 revolving. After a support is touched they immediately 

 coil around it and then begin to curl up somewhat after 

 the manner of a watch spring. This draws up the stem 

 of wliich they are a part. 



Examine the tendrils of wild grape, pea, Virginia 

 creeper, catbrier, white bryony, or any others you may 

 drilT'(T)^*are ^^^' ^^tice the position of the tendril on the main 

 modified sttp- stem and try to decide of what part of the plant it is a 

 ules (parts of modification. Notice the suckers or disks at the ends of 

 leaves); Th, the tendrils of the Virginia creeper. Can you discover 

 thorn. ' ' their use? 



Stems modified as Thorns. — Leaves and parts of leaves maybe changed 

 into thorns for the protection of the plant. In some instances the stem 

 becomes a spine or thorn. Such is the case in the honey locust. 



Compare it with the black locust, in which a part of the leaf, the stipule, 

 becomes the thorn. All such modifications seem to result in the better pro- 

 tection of tender parts wliich might otherwise suffer from the attack of 

 animals. 



