26 REPRESENTATIVE PLANTS 



until leaves are studied, and then insert here the purpose of 

 the green stem. 



Look at the hollow stem of the scouring rush and the 

 grasses. Offer, if possible, an explanation. What evidence 

 is there to you to show that thorns are really true stems or 

 rather branches ? Compare at this point with the rose or 

 prickly ash stems which are covered with prickles. What is 

 the difference between a thorn and a prickle ? What may 

 be the purpose of each ? 



Read in particular about cacti and the reason for their 

 fleshy stems and their leaves reduced to prickles. Consult 

 books that describe deserts and their plant life. 



After careful study and reading, seek some explanation 

 for the peculiar stems (like those of the cacti, euphorbias, 

 etc.) of so many desert plants. These plants, in their 

 peculiarities, strongly illustrate the effect of environment 

 or surrounding influences on the form and structure. Ecol- 

 ogy is the study of such influences and effects. 



Practical application. The government investigators are 

 doing much experimentally to find out what grains, grasses, 

 fruits, and trees will grow well in our dry West. They ob- 

 tain their new forms from every dry land on the earth. 

 Such forms are Durum and Speltz wheats, milo and Kaffir 

 corns, and alfalfa. 



What did Mr. Burbank do for the arid regions when 

 he produced a spineless variety of cacti? 



6. Structure of Stems 



Material. Various woody stem sections should be col- 

 lected, both di- and monocotyledons. Sumac and elder are 

 especially good for the former, and greenbrier, cornstalks, 

 and palm-leaf stalks for the latter. Material may be pre- 

 served in formaldehyde, if stems are to be studied in 

 winter. 



