ATROPHY OF ORGANS FROM LACK OF USE 265 



(2) Epicotyl and primary leaf of Nymphaea. 

 These structures in the water-lily are good 

 examples of normal atrophy in individuals. Dur- 

 ing germination (fig. 77) the cotyledons of the 

 water-lily remain inside the seed, and a new 

 organ (at right angles to them) grows vertically 

 upwards. The lower part of this is the first 

 internode of the stem (epicotyl), and the upper 

 part is a primary acicular leaf. It grows up- 

 wards through the mud until the summit of the 

 leaf reaches light. The growth of the epicotyl is 

 then much slower, and its terminal bud begins to 

 shoot out horizontally. The use of this growth of 

 the epicotyl and primary leaf is to carry the bud 

 to the light. When that purpose is achieved, these 

 structures atrophy. A similar occurrence may be 

 found in Sagittaria sagittifolia (fig. 40, H, I, p. 72). 

 In that case, however, it is the hypocotyl which elon- 

 gates, until light is reached, and then degenerates. 



(3) Roots of Utricularia, cotyledons of parasitic 

 plants, leaves transformed to spines in Phyllocactus 

 crenatus. As instances of atrophy throughout a 

 species produced by inutility of the parts con- 

 cerned, we have already mentioned the roots of 

 Utricularia and the cotyledons of the parasitic 

 plants Cuscuta, Orobanche, etc. 



The spines of Phyllocactus crenatus produced . 

 from modified leaves are another example (fig. 78). 

 Above the rounded base by which a branch of 

 Phyllocactus is attached to older branches, there 



