182 APPLIED BIOLOGY 



observed twine in the direction the watch-hands move (i.e., 

 clockwise, west-north-east) ; or in opposite direction (counter- 

 clockwise, east-north-west). Plants of some species appear 

 to move always in one of these directions. 



175. Creeping Stems. In walking across pastures, sandy 

 fields, and other places where tall plants do not grow, look 

 for plants which have their stems prostrate on the ground. 

 Do you see that such plants have any advantage over erect 

 tall stems under the conditions in which*they live? 



176. Underground Stems. Many 

 plants have their main stems under- 

 ground. Three common forms of 

 such stems are rootstocks, tubers, and 

 bulbs. 



Rootstocks. These are root- 



FIG. 53. Underground 1-1 i_- i_ j.u *i 



stem of Solomon's seal. llke stems whlch S row m the SOlL 



a, terminal bud ; b, c, d, Many ferns, grasses, sweet flag, golden- 

 ye "Troote' '""^ rod ' quack-grass, peppermint, iris, 

 Solomon's seal, and trilliums are ex- 

 amples of common plants with rootstocks (rhizomes). That 

 these are not true roots is shown by the presence of buds 

 and by the formation of branches and leaves. Solomon's seal 

 (Fig. 53) is especially good for study. Its rootstock sends up 

 an erect branch every spring which becomes the above-ground 

 stem with leaves. All the above-ground part of the plant 

 is herbaceous and dies in autumn. The seal-like scars on the 

 rootstock mark the positions of the above-ground branches 

 in successive years. The oldest portion of the rootstock 

 dies and decays ; and some plants have short rootstocks, be- 

 cause the part more than a few years old has died and decayed. 

 Tubers. Many of the rootstocks mentioned above store 

 food, but do not undergo noticeable thickening at any 

 particular point. In some plants storage of food in stem 

 underground causes great enlargement and produces tubers 

 (e.g., the common potato and Jerusalem artichoke). 



