THE STRUCTURE AND WORK OF THE STEM 113 



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 sup- 

 port in a given direction, some revolving with the course of the sun, 

 others in the opposite direction. When such a stem touches an object 

 during its first growth, it is im- 

 mediately stimulated to turn 

 toward the object and coil around 

 it. 



Leaves and Stems modified as 

 Holdfasts. — In the common nas- 

 turtium {tropwolum) the leaves 

 revolve in much the same man- 

 ner as do the stems mentioned 

 above. This movement results 

 in some of the leafstalks fasten- 

 ing around supports, thus draw- 

 ing the stem up. 



Tendrils. — In some plants 

 definite climbing organs, known 

 as tendrils, are developed. A 

 tendril, which has the appear- 

 ance of a much twisted stem, 

 may be modified 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 pos- 

 sible, 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 which they are a part to the support. 



Stems modified as Thorns. — Leaves and parts of leaves may be 

 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. 



In the case of the black locust, a part of the leaf, the stipule, becomes 

 the thorn. All such modifications seem to result in the better protection 

 of tender parts which might otherwise suffer from the attack of brows- 

 ing animals. 



A honey locust ; the thorns are modified 

 branches. 



HUNT. E8. Bio. 8. 



