32 



THE INDIANA WEED BOOK. 



Fig. 7. Fibrous roots of red clover showing the 

 nitrogen storing nodules. (After Piper.) 



nodules caused by bacteria. 

 (Fig. 7.) These bacteria have 

 the power to fix the free nitro- 

 gen of the air and develop ni- 

 trogen salts. The clover plant 

 uses part of these nitrogen 

 salts as food and leaves some 

 of them in the ground. Weeds 

 of the pea family are thus to 

 some degree beneficial in that 

 they help enrich the soil. 



The roots of most biennial 

 weeds, as well as those of some 

 perennials, are often thick or 

 fleshy, being composed largely 

 of starch which has been stored 

 to be used in giving the flower 

 stems of the second or succeed- 

 ing year nourishment for a 

 quick growth in spring. Such 

 weeds often have one large central tap-root extending straight 

 downward, with a few smaller roots branching from its sides. The 

 roots of a weed extend downward or outward in search of a suf- 

 ficient supply of moisture; if this be lacking the weed, like all other 

 plants, ceases growth, shrivels and in time dies. 



The stems of weeds. — The stem is the main axis of the plant 

 and is supposed to bear the roots below ground and the leaves and 

 flowers above. Most stems of weeds are more or less branched, 

 some of them very much so; if not at all branched they are called 

 simple. If the stem dies down to the ground each year the plant 

 is called an licrb, or if it twines, an herbaceous vine. Almost all 

 weeds are herbs. Stems with a woody texture which survive the 

 winter above ground are woody vines, shrubs or trees. One shrub 

 and two woody vines are included in the list of Indiana weeds, viz., 

 the blackberry, poison ivy and trumpet-creeper, while the common 

 elder might with propriety also have been included. 



In structure stems of weeds and other flowering plants are di- 

 vided into two great classes. In one class, called endogens, or in- 

 side growers, the woody or vascular tissue is usually scattered in 

 bundles through the stem, and there is no visible distinction of 

 bark, wood, etc. By cutting across the stem of a dry cornstalk one 

 can readily find these bundles running like fibres lengthwise 



