194 PRACTICAL COURSE IN BOTANY 
6. Examine the leaves of the worst weeds that you know of and sea 
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- 
