144 BOTANY. 



186. — Frequently the new stems which are normally formed 

 make but a very little growth, and in perennials become 

 covered by the subsequently formed tissues ; they thus become 

 the so-called dormant buds. Under favorable conditions they 

 may resume their growth long afterward, and they are then 

 liable to be mistaken for adventitious stems. Probably very 

 many of the supposed cases of adventitious stems upon the 

 older stems of Dicotyledons are in reality only the late 

 growths of stems which have been dormant for a long time. 



(a) The development of stems may be studied in almost any plant. 

 Those which have large winter buds, bowever, oifer some advantages 

 to the beginner. Such are the buds of hickory, horse-chestnut, lilac, 

 etc. 



(6) Vertical sections should be made of the buds before they resume 

 their growth in the spring, and these should be compared with similar 

 sections made after some growth has taken place. 



(c) Many of the common annuals with a continued growth — t.g., 

 balsam, mallow, etc. — may be profitably studied for making out the 

 growth of summer buds. The young shoots of many shrubs — e.g., 

 elder and lilac — are also excellent for study. 



((?) Thin enough longitudinal sections should be made to show the 

 punctum vegetationis. The specimens may often be made much more 

 instructive by coloring with carmine, or other staining fluids. 



§ III. Of Leaves \'s. General. 



187. — Every leaf originates in the Primary Meristem of 

 the punctum vegetationis. It is at first a small projection 

 or papilla, composed of one or more cells, which undergo a 

 rapid division, thereby producing the quick early growth 

 before mentioned (p. 139). Generally the multiplication of the 

 cells is such as to give rise to a surface whose plane cuts the 

 stem transversely. In many cases the apex of the leaf soon 

 becomes changed into permanent tissue while the base con- 

 tinues to grow, indefinitely in grasses and many other 

 Monocotyledons, and definitely in most Dicotyledons. In 

 other cases the base passes over into permanent tissue, while 

 the apical portions keep on growing, as in ferns and some 

 pinnate leaves of Dicotyledons. 



188. — Many leaves are raised upon a stalk by a subsequent 

 growth between the stem and the base of the leaf ; this leaf- 



