194 PRACTICAL COURSE IN BOTANY 



6. Examine the leaves of the worst weeds that you know of and see 

 if these will help in any way to account for their persistency. 



Field Work 



(1) In connection with Sections I and II, observe the effect of the lob- 

 ing and branching of leaves in letting the sunlight through. Notice any 

 general differences that may appear as to shape, margin, and texture in the 

 leaves of sun plants, shade plants, and water plants, and account for them. 

 Study the arrangement of leaves on stems of various kinds, with reference 

 to the size and shapes of leaves and their light relations. Consider the 

 value of the various kinds of foliage for shade ; for ornament ; as producers 

 of moisture ; as food ; as insect destroyers, etc. 



Make a special study of the twelve principal deciduous trees of your 

 neighborhood. Compare the leaves, bark, and branches of the same 

 trees so that you will be able to recognize them by any one of these means 

 alone. 



(2) In connection with Sections III and V, consider the effects upon soil 

 moisture of transpiration from the leaves of forest trees and from those 

 of shallow-rooted herbs and weeds that draw their water supply from 

 the surface. Consider the value of forests in protecting crops from exces- 

 sive evaporation by acting as wind breaks. Study the effect of the fall of 

 leaves upon the formation of soil. In any undisturbed forest tract turn up 

 a few inches of soil with a garden trowel and see what it is composed of. 

 Notice what kind of plants grow in it. Note the absence of weeds and 

 account for it. Compare the appearance of trees scattered along windy 

 hillsides, where the fallen leaves are constantly blown away, or in any 

 position where the soil is unrenewed, with those in an undisturbed forest, 

 and then give an opinion as to the wisdom of hauling away the leaves every 

 year from a timber lot. 



(3) In Section VII, observe, in different kinds of leaf mosaics, the means 

 by which the adjustment has been brought about and the purpose it sub- 

 serves. Make a list of plants illustrating the two habits. Notice the form 

 and position of petioles of different leaves, and their effect upon light ex- 

 posure, drainage, etc., and the behavior of the different kinds in the wind. 

 Look for compass plants in your neighborhood, and for other examples of 

 adjustment to heat and light. Study the position of leaves at different 

 times of day and in different kinds of weather and note what changes occur 

 and to what they are due. 



Make a list of ten plants that seem to you to have best worked out the 

 problem of leaf adjustment, giving the reasons for your opinion. 



Study the drainage system of different plants and observe whether there 

 is any general correspondence between the leaf drainage and the root sys- 



