THE WORK OF STUMS 277 



stimulated in clockwise climbers, the east side in 

 counter - clockwise climbers. Why gravity should 

 stimulate the east side rather than the west (or vice 

 versa) we do not know, any more than why it should 

 stimulate the lower more than the upper side in so 

 many cases. 



Many plants have underground stems (e. g., Iris, 

 Ferns, etc.). Such stems are advantageous in many 

 ways; the plant may die down in the fall and come 

 up again from these stems in the spring (in such cases 

 food is stored in them [Potato, etc.] which enables 

 the plant to get a good start in the spring) ; they 

 enable the plant to spread and take slow but sure 

 possession of a great area (e. g., Horse-tail, Mints, 

 Grasses, etc.); they are indispensable to the plant 

 in sand-dunes and similar places, since they bind the 

 soil together and keep it from blowing away.^ It is 

 interesting to note that these underground stems, 

 whose work is similar to that of the roots, closely 

 resemble roots in their general appearance and are 

 commonly mistaken for them. This affords another 

 illustration of the fact that function determines form 

 and structure. If we force underground stems to grow 

 up into the light and air, they begin to function like 

 ordinary stems and assume, in a measure, the ordinary 

 appearance and structure of stems. 



^ Plants which have this habit ai-e of great importance for binding the soil 

 of dikes, levees, banks of canals, etc. 



