REPRODUCTION 357 



develop an independent existence. It may be observed in its sim- 

 plest form in many perennial plants, whose rhizomes develop at one 

 end and die off and decay at the other. This is commonly seen in 

 the iris, the lily-of-the- valley, and many other plants. If a rhizome 

 of this kind had formed branches, the parts which formerly joined 

 the separate branches may eventually rot away and the branches 

 then develop an independent existence. 



Besides this indefinite form of propagation, one may observe in 

 nature the formation of special organs serving the same purpose. 

 Many plants produce long horizontal shoots, stolons or runners, 

 either above or below the ground. These bear peculiar buds which 

 root readily and attain a completely independent existence as 

 soon as the connecting part of the stolon has been subjected to 

 decay. A good illustration of such plants Ls the strawberry 

 which usually produces several "runners." Bulbous plants often 

 form lateral buds in the axils of their scalelike leaves, which can 

 develop into independent bulbs. Small bulbs and tubers also 

 may grow on various aerial parts of a plant, for instance in the 

 leaf axils, as in Ficaria and certain lilies. In many varieties of 

 onions they replace flowers in the inflorescence. The majority of 

 these special organs of vegetative reproduction are characterized 

 by an accumulation of various amounts of stored substances and by 

 having a very pronounced rest period. In some viviparous plants, 

 such as onion or meadow grass these bulbs germinate on the parent 

 plant and, after dropping to the ground, continue their growth. 



The conditions under which the organs of vegetative propaga- 

 tion are formed in higher plants have not been studied sufficiently. 

 As a general rule, the most favorable conditions seem to be those 

 that are most conducive to vegetative growth, namely, a high 

 atmospheric humidity and weak illumination. Evidently, these 

 conditions are exactly the opposite of those which promote sexual 

 reproduction; hence, special organs of vegetative propagation are 

 found chiefly in plants growing in damp, shady places, or in water. 

 This type of propagation, therefore, may be regarded as a special 

 supplement to or substitution for sexual reproduction. 



Notwithstanding its long history, the question as to the exact 

 role played in the life of a plant by each of these two methods of 

 multiplication has not yet been finally solved. Likewise, it has 

 not yet been determined whether plants may be propagated 

 vegetatively an indefinite number of generations without the help 



