STUDIES OF SEED-PLANTS 191 



In field trips taken in connection with this course in biology, 

 attention should be directed to the arrangement of leaves in the 

 light relation whenever good specimens are found. The discus- 

 sion of the above outline of the most important types of leaf- 

 arrangement should be taken up in the laboratory with specimens at 

 hand to illustrate the main points, and pupils should make outline 

 sketches showing various types of leaf-arrangement. 



Reading for pupils: Chapter II, "Foliage Leaves: the Light- 

 Relation," in Coulter's "Plant Relations." The same chapter is in 

 "Plant Studies" and "Plants" by the same author. Or Chapter 

 X in Bergen' s " Foundations of Botany," or Chapter XI in Bergen 

 and Davis's "Principles of Botany." 



183. Special Adaptations of Leaves. While the primary 

 purposes of leaves are starch-making, transpiration, and 

 breathing, many leaves have assumed additional work for 

 which their structure shows special adaptations. The most 

 interesting of these are those concerned with catching insects, 

 storing food, climbing, and protection against animals (leaves 

 modified into prickles). 



Leaves as Insect Traps. Certain plants growing in 

 bogs have curious leaves adapted to catching insects. Some 

 of these plants have leaves in the form of deep cups or pitchers 

 containing water in which insects are drowned (pitcher 

 plants) ; some species (the sun-dews) have strong bristles 

 coated with a sticky substance which holds fast any insects 

 which happen to touch them (Fig. 57) ; and a third kind 

 (Venus fly-trap) have at the end of each leaf a folded structure 

 which opens and shuts very much like a steel-trap (Fig. 2). 

 In all these cases the insects caught either decay by action 

 of bacteria or are digested by certain fluids secreted by the 

 leaves, and are absorbed by the cells of the plant. Thus 

 the ordinary food-supply of these plants is supplemented 

 by animal food, and such plants are called carnivorous 

 (flesh-eaters), or sometimes insectivorous (insect-eaters). 



Leaves for Climbing. The leaf-stalk or petiole acts as 

 a tendril in some plants (nasturtium, clematis, etc.). In 



