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 the same fact can be obtained from many crevice plants. 

 These plants send their root systems into the crevices of 

 rocks, and spread a rosette of leaves against the rock face. 

 In the next year a new rosette of leaves, developed further 



FIG. 89. Section through the leaf of a quillwort (Isoetes), showing the four large air 

 chambers (a), the central vascular region (6), and the very poorly developed 

 cortex. 



up the stem, is also found against the rock face. It is 

 evident that the stem has been pulled back into the crevice 

 enough to bring the new leaves against the rock, and this 

 pulling has been effected by the new roots, which have 

 laid hold of the crevice soil, or walls. 



(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- 



