ECOLOGICAL CROSS SECTION OF THE MISSISSIPPI. 



161 



Forest formation. 



In the deciduous forests 



dominant. The 



most abundant as well as the most 



black oak, Q 



coccinea tinctoria. Associated with it 



Q. wibricaria, Q, Muhlenhergii, Q. nigra, Q. rubra and Q 



Q 



formm 



of the undergrowth. Interspersed with the oaks are hickories, 

 Carya olba, C. pordna odorala and C. tomentosa* ^ 



Other 



make up the forest are Jugi 

 fulva, Sassafras officinale, A 



inum, Fraxmus amencana, Morus rubra ana JJiospyros vir- 

 giniana. Though the tree stems are placed at a consider- 

 able distance from each other, their foliage forms a compact 

 roof, allowing but little light to penetrate. Measurements 

 made during the first days of July, about the noon hour, 

 showed the intensity of the diffused light in the forest 

 to be from 8 to 10% of that of the total daylight, 

 the sky being free from clouds. As a result of the low 



occurs but little 



light there 



intensity of the diffused 



undergrowth of a shrubby nature, though specimens of 



Q 



stellata. 



amencanus 



R. glabra, Prunus serotina, Pyrus 



__^.. and 



Cornus asperi- 



met 



folia and C. florida, Samhucus canadensis and Viburnum 

 prunifolium as well as of Diospyros virginiana 

 with. Also as a result of the small amount of light the 

 shrubs do not form thickets here but occur as 



more 



ground 



less isolated individuals. The 

 of biennials and perennials which develop their root leaves 

 either before the leaves of the deciduous trees have ap- 

 nparpH or before thev have attained their full development;! 



* Illustrated byTrelease in: Juglandaceae of the United States. Rept. 



Mo. Bot. Card. 7 : 25. 1896. 



t Biennials, as shown by Wiesner in his Der Lichtgenuss der Pflanzen, com- 

 mence their development during a period of decreasing light intensity, i. e. 

 during the latter part of the year and, after a period of rest, resume growth. 

 Perennials, possessing a store of food material, require less light than do an- 



nuals. 



predomina 



Hydrastis, Sanicula, Spiraea^ Arisaema, etc. In woods many of the annuals 

 have made their appearance l>efore the leaves of the trees arc unfolded. 



11 



