94 



PLANT RELATIONS. 



"layering," by which a stem, as a bramble, is bent down 

 and covered with soil. The covered joints strike root, and 

 the pulling follows (see Fig. 86). A very plain illustration 

 of this pulling by roots can be obtained from many tuberous 

 plants. Tubers, bulbs, rootstocks, etc., are underground 

 structures which have been observed to bury themselves 

 deeper and deeper in the soil. This is effected by the young 



Fig. 89. Section through the loaf of a quilhvort (Isoetes), Bhowing the four large air 

 chambers (a), the central vascular region (b), and the very poorly developed cortex. 



roots which they continue to put forth. These roots grip 

 the soil, then contract, and the tuber is pulled a little deeper. 

 The compact tuber known as the Indian turnip (" Jack-in- 

 the-pulpit ") has been found to bury itself very deeply and 

 rapidly, and this may be observed by transplanting a young 

 and vigorous tuber into a pot of loose soil. 



(3) Soil dangers. — In this connection certain soil dan- 

 gers and the response of the roots should be noted. The 

 soil may become poor in water or poor in certain essential 

 materials, and this results in an extension of the root sys- 



