58 THE PLANT WORLD. 



whose thorns have brought many a sinner to repentance. The Brier 

 family {Sinilacece) though closely allied to the foregoing families, is 

 not of sufficient interest to warrant more than a passing comment. 

 Its chief member is Smilax, which contains upwards of 200 species, 

 mainly tropical. The plants are nearly all shrubby vines, the stems 

 often prickly, the flowers small and inconspicuous in umbellate clus- 

 ters borne in the axils of the leaves. The sexes are separate, that is, 

 the staminate flowers are borne on one plant, and the pistillate on an- 

 other; the fruit is a berry. 



In the next paper I shall discuss the Irises, the Amaryllids and 

 their relatives. 



INFLUENCE OF LIGHT ON THE FORM OF PLANTS. 



THE effect of the light-supply in determining the form of plants 

 is well recognized. If a grass seed germinates under a tub, 

 the little plant does not spread out at random in its growth. 

 If the edge of the tub is raised to admit a ray of light, instead 

 of growing upright, as it naturally would, the shoot bends toward the 

 ray of light and grows by the shortest path to the opening. After 

 passing into the full light it develops its normal form. The form and 

 direction of growth of every branch is determined largely by the ac- 

 cessibility of light. The form and structure of every leaf also are 

 just as largely dependent upon the light-supply. Innumerable differ- 

 ences in the shapes of individuals of the same species are caused by 

 the struggle to obtain light. Branches develope in positions where 

 their leaves can be unfolded to the light with the least obstruction. 

 The natural round and full symmetry of the tree grown in an open 

 place is due to the unobstructed action of the light on branch devel- 

 opment. Trees have developed their habit of lofty growth by a con- 

 tinuous struggle to secure light. The same inciting cause has led to 

 the development of the habit of twining in certain plants. Other 

 plants have so modified their structure that they are able to secure 

 sufficient light in the shade of the forest. Sleep movements of plants 

 are other well-known reactions of light. In cultivation, the necessity 

 of light is well recognized, and our plans for planting fields and gar- 

 dens are made with reference to the plants used in order to secure the 

 necessary light and nutrition for the best development of each indi- 

 vidual. — Herbert J. Webber in " Year-book of the U. S. Departmetit of 

 Agricultu re for i8g6." 



