56 



PROPAGATION OF PLAXTS. 



has been explained elsewhere, but may be referred to 

 again for the purpose of noting the exceptions to the 

 rule, as well as to explain more fully some peculiarities 

 of stems in their embryonic stages. 

 In the young seedling Oak, shown in figure 14, the 



two cotyledons or seed-leaves 

 are attached to one side of 

 the stem and remain partly 

 enclosed within the shell, 

 this being the usual position 

 in which they are found on 

 the starting plantlets, for they 

 seldom expand sufficiently to 

 free themselves from their 

 horn-like covering ; neither 

 is it necessary, for the secon- 

 dary leaves are early de- 

 veloped, and before the nu- 

 triment stored up in the nut 

 is entirely exhausted in pro- 

 ducing the plumule and 

 simple root. A similar re- 

 stricted development of seed- 

 leaves occurs in nearly all of 

 the nut-like seeds, and even 

 in the seeds of many herbs. 

 The common garden Pea 

 is a familiar example of a di- 

 cotyledonous seed, the seed- 

 come up," or appear above- 

 ground on the ascending stem ; but on the closely allied 

 garden Bean the cotyledons or seed-leaves are always con- 

 spicuous objects on the young plants. This is also true 

 with many kinds of trees, such as the Pear, Apple, Plum 

 and G'herry among fruits, and the Ash, Elm and Maple 

 among, our most familiar forest trees ; as soon as the 



Fig. 15. SEEDLING MAPLE 



leaves of which do not 



