306 CONSERVATIVE ORGANS. [LECT. VII. 



preserve until they are lost in a mucilaginous sub- 

 stance, secreted under the detached epidermis. 

 A layer of new ligneous cords is thus applied over 

 those of the old stem, and the development of 

 other branches still adding fresh layers, the stem 

 is gradually increased in diameter. M. Aubert 

 du Petit-Thouars thinks that, for the formation of 

 branches in these plants, a vital point (un point 

 vital) exists at the axilla of each leaf; but re- 

 mains latent and inactive, unless peculiar circum- 

 stances occur to call it into activity ; and this, he 

 conceives, constitutes the difference between these 

 gems and the buds which appear in the axillae of 

 the leaves in the great majority of dicotyledonous 

 plants. I shall, however, soon have an oppor- 

 tunity of demonstrating to you that the same 

 circumstance occurs in dicotyledonous stems ; on 

 which buds sometimes appear, that have existed 

 in a latent state for many years; and can be 

 traced back to their origin in the change of or- 

 ganization, occasioned by the frustrated effort to 

 develop them, in the successive layers of wood 

 which have annually added to the diameter of the 

 stems, on which they are ultimately developed. 



The development and growth of the herba- 

 ceous, solid, monocotyledonous stem is nearly the 

 same as that of the ligneous, except that the parts 

 are more rapidly evolved. If we trace the growth 

 of the stem of the White Lily, for example, 



