Minnesota Plant Life. 



429 



so constructed that they can exist with much less illumination 

 than others. This is particularly the case if they are devoid 

 of leaf-green, and certain fungi are able to grow and mature 

 their fruit-bodies in absolute darkness. The ordinary green 

 plant must, however, be supplied with illumination sufficient 

 for the use of the starch-making machinery, for leaf-green may 

 be described as a variety of light-engine. Hence there arise, 

 especially in the leaf-areas of plants, a variety of adaptations by 

 means of which the leaves are spread out to the sun. If a plant 



FIG. 213. Elm tree growing in the open. I^ight is received on all sides. After 

 photograph by Williams. 



is growing where it is not shaded by other plants, it may assume 

 the mat or carpet habit of growth ; but if it is shaded by other 

 plants it must become taller in order to get its share of illumina- 

 tion. The positions of leaves on the plant body are, in the 

 economy of the plant, often very carefully adjusted, so that one 

 leaf fits quite exactly into the spaces between other leaves. 

 Thus, what are known as leaf-mosaics are formed. The leaves 

 sometimes make a rosette at the base of the fruiting stem, a 

 condition that may be seen in a dandelion, in the evening- 



