STUDIES OF SEED-PLANTS 181 



disks at the tips. If possible, observe the growth and 

 movements of tendrils on some plant which can be watched 

 from day to day. 



Leaves of certain plants, as those of the pea, form tendrils ; 

 but these are easily distinguished from branches because they 

 are connected with leaves and not in the 

 usual positions of branches. 



173. Branches as Thorns. Examina- 

 tion of specimens of thorns from trees of 

 pear, honey-locust, hawthorn, buck-thorn, 

 and others shows that they are branches, 



or tips of branches. Prickles on many F 52 ^ eaf and 

 plants are simply elevations of the bark. stem-tendrils (/) 

 Peel off some bark from stems of blackberry of Ampeiopsis 

 or rose and note that the prickles are not 

 attached to the wood as branches of stems are. Barberry 

 prickles are modified leaves, and those of black locust are 

 stipules of leaves. 



The uses of thorns and prickles are sometimes doubtful. 

 While they appear to be adaptations for protection against 

 grazing animals, it is well known that often they do not so 

 protect. Cows and sheep often eat the thorny branches, 

 especially young and delicate shoots when the thorns or 

 prickles are soft. Certainly these sharp structures are not 

 absolutely necessary, for many plants without them appear 

 to flourish as well as their " armed " relatives. It is probable 

 that prickles and thorns developed first without any ref- 

 erence to defensive use (e.g., the spines of cacti, which are 

 leaves reduced in adapting to dry climate), and that now 

 they may sometimes be of slight advantage to the individual 

 plants that possess them. 



174. Twining Stems. Some stems climb without the aid 

 of tendrils. Study plants of pole-beans, morning-glory, hop, 

 on any wild plants available. Lay your watch on the ground 

 with the hour-mark pointing north, and note whether plants 



