492 PRACTICAL BOTANY 



plants flower at the expense of food material that was 

 deposited in former seasons (Sects. 34 and 69), and do most 

 of their vegetative work before the leaves of the trees are 

 developed so as to shade them. As the season advances 

 and the light intensity increases, these low plants may still 

 work by means of the diffuse light that is filtered through 

 the tree tops. 



Any local forested area will afford students interesting 

 studies in the strata of forest vegetation. 



448. Fractional part of total sunlight required by various 

 plants. 1 The proportion of the full strength of sunlight re- 

 quired by any given species is not the same in different lati- 

 tudes ; for example, the common dandelion at Vienna (lat. 

 48.12) may grow in an illumination of one twelfth, but in 

 northern Norway (lat. 70.33) the total light of the sun is 

 needed to enable it to grow. 



The same individual often requires different amounts of 

 light during different stages of its development. The English 

 ivy (Hedera) will not bloom with a light intensity less than 

 two ninths of total daylight, and therefore flowers are never 

 seen on ivies grown as house plants. But the vegetative 

 organs continue to grow with an illumination as low as one 

 forty-eighth. 



Among the seed plants which can flourish in deep shade 

 are many species of epiphytes and lianas. Good examples of 

 the former are many tropical orchids ; of the latter, one of the 

 most familiar is the common frost grape (Vitis cor dif olio), 

 which can grow with an illumination as low as one seventieth. 

 On the other hand, plants which live in the open and compara- 

 tively unshaded, like all the larger prairie species, such as many 

 kinds of Liatris, Coreopsis, sunflowers, rosinweeds, some iron- 

 weeds and wormwoods, require high illuminations to thrive. 



1 See Wiesner, Der Lichtgenuss derPflanzen (Engelmann, Leipzig). Great 

 caution must be used in drawing conclusions on this head, since lack of soil 

 water and salts may easily be responsible for part of the effects attributed to 

 scanty light supply. 



