450 PLANTS AND THEIR RELATION TO MAN 



cause itching, swelling, and eruption, if they are merely touched, 

 Jimson weed is particularly dangerous because it grows abun- 

 dantly in city lots. Children have been made seriously ill by 

 chewing this weed. It is a bushy plant with large leaves and 

 white or purplish flowers. In the late summer it has con- 

 spicuous dry fruits full of small seeds. Poison ivy grows along 

 roadsides, climbing over fences, and up tree trunks, to which 

 it attaches itself by tiny air roots. Its leaves (Fig. 317) are 

 deeply notched or three parted, and in spring and summer are a 

 beautiful green color. In late summer they turn to a handsome 

 deep red, and by their brilliant coloring lure many an unsuspect- 

 ing person into plucking them. But poison ivy, no matter how 

 attractive its appearance, should never be touched. 



The uses of stems to the plant. Leaves cannot make food 

 unless they are exposed to the sunlight and unless they receive 

 an abundant supply of water. Stems, trunks, branches, and 

 twigs are useful to the plant because they lift the leaves up- 

 ward and outward to the light, and because they carry to 

 the leaves the soil moisture taken in by the root hairs. They 

 are equally useful to the plant in carrying to the roots the food 

 manufactured by the leaves. Two currents of sap constantly 

 flow through stems : one upward from the roots, and the other 

 downward from the leaves. 



The structure of steins. The structure of a stem can be 

 studied by examining a young maple or lilac stem which has 

 been cut in halves lengthwise. The split stem shows three 

 distinct portions : first, an outside covering, the bark ; second, 

 a woody portion, the wood; and third, a pithy portion, the 

 pith, at the center (Fig. 318). The bark is extremely important 

 and, when examined closely, is seen to consist of three or 

 more distinct layers, a dry brown layer which is the outermost 

 covering of the stem, a green layer which lies directly beneath 

 the brown, and a tough fibrous layer, the bast, which lies next to 



