PLANT STUDIES 
"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 qnillwort (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- 
