182 PLANT PHYSIOLOGY AND ECOLOGY 



on the framework in order to serve as an opening. Ordinarily it is not 

 necessary to ventilate each tent, but, if the humidity within becomes 

 too high, this is effected by means of hoods at the top or sides. 



Practically all flowering plants will show some adaptation to the 

 different light intensities of the shade tent. Helianthus, Allionia, 

 and Taraxacum have been found especially suited for this work, though 

 a number of others, Bursa, Galium, Onagra, etc., are equally good. 

 Helianthus annuus (wild form) illustrates fairly well the behavior of a 

 stable species, and Allionia linearis or A. nydaginea that of a plastic 

 one. Taraxacum taraxacum shows the effect of shade upon divided 

 leaves and the form of the rosette. Both Bursa and Onagra are espe- 

 cially good to show the stretching of the petioles and internodes. It 

 is best to start the seedlings under normal conditions and then to place 

 4-6 of each species in each tent as soon as they begin vigorous growth. 

 A set should likewise be left in normal light to serve as checks. 



During the growth of the plants make occasional readings of light 

 intensity for the three tents, and make one determination of the starch 

 content of a representative leaf of each species for each tent. Follow 

 with care the differences in the growth and behavior of each species 

 in the three tents and in the sunlight. When the plants are well grown, 

 make an outline drawing of the leafy plant of each species for the four 

 conditions and a similar outline of a representative leaf of each, drawing 

 all stems as well as leaves to the same scale. Select a representative 

 leaf from each form, and make a leaf print for the four forms of each 

 species. Kill a similar set of leaves from each species and make a 

 study of the structural modifications as shown by microtome sections. 

 A similar study of modifications in the number of stomata may well 

 be made by stripping the epidermis from the fresh leaves. Prepare a 

 concise account of the adaptation of the species concerned to different 

 light intensities. 



198. Types of leaves as determined by light. Isophotic 

 leaves are equally illuminated on both surfaces, or nearly so, and 

 possess a more or less uniform chlorenchym. Diphotic leaves 

 are unequally illuminated, and show a division into palisade and 

 sponge tissue. The ordinary horizontal or dorsiventral leaves 

 are usually diphotic. Leaves of this type contain both palisade 

 and sponge, though the relative importance of the two varies 

 considerably in different species. Diphotic leaves are character- 

 istic of sunny swamps, meadows, prairies, etc., and are frequent 

 in xerophytic habitats. Floating leaves, in which the light is 

 almost completely cut off from the lower surface, are also diphotic. 

 This type of structure is often found in the leaves of shady 



