J 92o] SMITH—BULBILS OF LYCOPODIUM 435 



in autumn in preparation for the next year, and continue to form 

 when growth is renewed in the spring. Then, beginning some time 

 in June, for more than two months the apex develops a succession of 

 sterile leaves, in the midst of which two to four bulbils may make 

 their appearance, always near each other and on one side of the stem, 

 on that side of a branch which is farthest from the other branch. 

 The primordia of the bulbils increase rapidly in size, pointing 

 vertically upward at first like the young leaves, and then gradually 

 becoming horizontal. They reach maturity in September. Since 

 the bulbils are formed but once a year and their bases are persistent, 

 it follows that they furnish a means of measuring the annual 

 increment of growth in length. There are other but less accu- 

 rate means of estimating a year's growth, by the old sporangia 

 whose walls do not completely disappear for several years, and 

 also by a difference in the length of sporophylls and sterile leaves. 

 The annual growth in plants bearing bulbils can be determined 

 quite easily for the last ten or twelve years, and in some cases 

 even for twenty years. It is surprisingly small, averaging about 

 one inch. 



The possibility of determining the age of any given part of the 

 stem leads to some interesting observations. For instance, it can 



V 



shown that the metaxvlem 



differential 



The cortical zones, 



most Lycopodium stems, are not differentiated 



until the third year. It is also possible to calculate the frequency 



dichotomy of the stems. The commonest interval 



three 



five, and six years also occur frequently. I have never found a 

 stem branching in two consecutive years. 



Like several other species, L. lucidulum has the old stem 



and the 



means 



part is six or seven years old. Observations from year to year 

 show that the plants, although adding annually one inch to their 

 length, maintain the same average height. Each year a part 



of the erect 



ing 



and finally horizontal. Fig. 20 represents the situation in four 



