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BOTANY 



than many other plants. Compare the spines of the honey locust, black 

 locust, and barberry. Look for leaf traces and buds on the stem, and decide 



from the relative position of the thorns and 

 these structures which of the above-named 

 structures are modified leaves. (Sometimes 

 a spine may be part of a leaf, as the stipule.) 

 Cactus. — In the prickly pear cactus, 

 notice that above the spines are little buds. 

 The position of the bud shows the spine to 

 be a modified leaf. What reason can you 

 give for this modification of the leaf of the 

 cactus ? How is the plant body modified to 

 meet the conditions of life in a desert? 

 Note the thickened stem. 



If a cactus is cut open, it will be found 

 to contain a very considerable amount of 

 water. The Indians of the New Mexican 

 desert region, when far from a source of 

 water, sometimes cut off the top of a large 

 cactus, mash up the soft interior of the 

 thickened stem, squeeze out the pulp, and 

 thus obtain several quarts of drinkable 

 water. 



Protection by Hairs. — In the mul- 

 lein, one of our hardiest weeds, the leaf is 

 covered with a coating of finely branched 

 hairs. Might such a covering be of use to 

 the leaf? In what ways? 



Leaves modified for Use in Climbing. 

 — Sometimes, as in the leaf of the pea, a 

 part of the leaf is modified for the purpose of climbing. In this case 

 a part of the leaf, called the tendril, becomes especially sensitive to the 

 stimulus of touch, and upon touching an object coils around it. Almost 

 any part of the leaf, or indeed the entire leaf, may be modified to become a 

 tendril. What part of the leaf of the pea here forms the tendril? If 

 material can be obtained, work out the morphology of modified parts of the 

 clematis; wild grape; Virginia creeper. 



Storage of Food or Water in Leaves. — Leaves may be modified for 

 the storage of food or water. Test an onion, which is a collection of thickened 

 leaves closely wrapped to form what is called a bulb, for starch, sugar, and 

 proteid. Squeeze the leaves of the Sedum and notice the water contained 

 in them. The Agave is a desert plant in which the leaves have become 

 greatly thickened as a water and food storage. Make a list of any plants 

 you know, as the cabbage, that store food in the leaves. 



Reduced Leaves. — Leaves may be reduced to scales or lost altogether. 

 In the asparagus what seem to be tiny leaves are branches which spring 



A cactus, showing the leaves modi 

 fied into spines. 



