52 STKUCTURE AND PHYSIOLOGY OF SEED PLANTS 



2. Is the leaf bilaterally symmet7ncal; i.e. is there a middle 

 line running through it lengthwise, along which it could 

 be so folded that the two sides would nearly coincide ? 



3. Is the leaf dorsiventral; i.e., has it distinct upper and 

 under surfaces ? 



4. Notice that the leaf is traversed lengthwise by a strong 

 midrib and that many so-called veins run from this to 

 the margin. Are these veins parallel ? Hold the leaf up 

 towards the light and see how the main veins are con- 

 nected by smaller veinlets. Examine with your glass the 

 leaf as held to the light, and make a careful sketch of 

 portions of one or two veins and the intersecting veinlets. 

 How is the course of the veins shown on the upper sur- 

 face of the leaf? 



5. Examine both surfaces of the leaf with the glass and look 

 for hairs distributed on the surfaces. Describe the man- 

 ner in which the hairs are arranged. 



40. The maple leaf. 



A. Sketch the leafy twig. 



1. How are the leaves arranged ? 



2. How are the petioles distorted from their natural posi- 

 tions to bring the proper surface of the leaf upward toward 

 the light ? 



3. Do the edges of these leaves show larger spaces between 

 them than the elm leaves did ; i.e. would a spray of maple 

 intercept the sunlight more or less perfectly than a spray 

 of elm ? Pull off a single leaf and sketch its lower sur- 

 face, about natural size. 



4. Of the two main parts (blade and petiole), which is more 

 developed in the maple than in the elm leaf ? 



B. Describe : 



i. The shape of the maple leaf as a whole. To settle this, 

 place the leaf on paper, mark the positions of the extreme 

 points, and connect these by a smooth line. 



