20 PRELIMINARY STUDIES. 



15. Examine the arrangement of the leaves on the 

 plants mentioned in Number 13. The points on the stem 

 where the leaves originate are called nodes; the spaces 

 between the nodes are internodes. On these or on other 

 plants find cases where the arrangement is opposite, i.e., 

 two at each node, one opposite the other; cases where 

 the arrangement is alternate, i.e., only one at each node ; 

 other cases where the leaves are in whorls, i.e., several at 

 each node, arranged around the stem. Examine as many 

 common house and outdoor plants as possible. The posi- 

 tion of the leaves of the previous summer may be seen on 

 the winter stem by the leaf scars, one at the base of each 

 bud that has formed for the following season. What 

 arrangement do you find to be the most common? 



16. Select twigs on which the leaves are arranged alter- 

 nately. Starting with any leaf, wind a string around the 

 twig so as to pass by the base of each leaf. Determine 

 whether the number of leaves passed in each revolution 

 about the stem is constant. Do you find the arrangement 

 the same in all the plants examined ? State what number 

 of leaves you find in one revolution about the stem in 

 each different case. Select a straight stem, and with a 

 knife cut a line vertically down from the base of each 

 leaf found in one revolution about the stem. What por- 

 tion of the circumference of the stem lies between each 

 two consecutive leaves ? Is this fraction the same in all 

 cases examined? The arrangement of the leaves on the 

 stem is called phyllotaxy. 



17. Consider the parts of a typical leaf, as of the Straw- 

 berry, Wild Geranium, House Geranium, Rose, Red Clover, 

 Apple, etc. The broad, expanded portion is the blade or 

 lamina ; the stem supporting the blade is the petiole or leaf 

 stalk; at the base of the petiole are the stipules. Find 

 leaves that have no petioles ; such leaves are said to be 

 sessile. Find leaves that have no stipules. Find cases 



