20 ELEMENTARY BOTANY. 



ground stems of the Mint illustrate the former, and the Irish 

 Potato the latter. A bulb is a very short stem or root-stock, 

 covered by bases of leaves in the form of thickened scales, and 

 bearing roots below. A corm is likewise short and thick, 

 bearing roots below, but destitute of scales. The Onion is a 

 familiar example of a bulb ; the Indian Turnip and Cyclamen 

 furnish examples of the corm. These thickened stems and 

 scales are reservoirs of nourishment. 



3. Some stems, as the corn-stalk, have woody strands scat- 

 tered irregularly through the whole interior and commingled 

 with the pith (Fig. 1). Such strands are found, though not 



always so easily recognized, in stems of the 

 Lily, Spiderwort, the Orchids, Solomon's Seal, 

 Asparagus, Flag, Palms and many others. 

 The plants with stems of this kind are, with 

 very few exceptions, Monocotyls^ — that is, 

 they have but one cotyledon or seed-leaf to 

 ^^'^■■^- each seed. When the woody strands attain 



their full development, there is no further increase in the 

 thickness of the stem (except in Palms and a few others), 

 though the plant may continue to live for some time. This is 

 clearly seen for example, in the stems of Lily, Asparagus, Corn, 

 etc., where the thickness of the stem when a few inches or a few 

 feet high has reached its limit and thereafter the diameter re- 

 mains the same. The strands in such stems are, as shown by 

 microscopic examination, composed of only two kinds of 

 tissue, namely, wood and bast. 



4. If a transverse section of a stem of Bean, Bindweed, 

 Mint, Sunflower, Nightshade, Buttercup, Dock, etc., be made 

 and examined with a lens, a central pith will invariably be 

 found, though sometimes it is torn, making the stem hollow. 

 Surrounding the pith but often indistinctly seen, are the iso- 

 lated woody strands, usually few in number, forming a circle 

 (Fig. 2). Dicotyls (with but few exceptions) have such stems.' 



' In older books the terms endogenous ("inside-growing") and exogenous 

 ("outside-growing") have hcen erroneously used to designate the mono- 

 cotyledonous and dicotyledonous stems, 



