392 PRACTICAL BOTANY 



of plants of successive ages will demonstrate that the 

 compressed cylindrical stalk results from the thicken- 

 ing of the midrib, and decomposition of the lateral wings. 



3. If this stalk be traced upwards it will be seen to 

 branch repeatedly, while on tracing the branches also 

 upwards they gradually assume the flattened form with 

 serrate margin, thus confirming the conclusions which 

 may be drawn from a comparison of younger plants, 

 viz. that the whole thallus, when young, was of a 

 flattened form, and that after growth and repeated 

 branching the lower portions assumed the flattened 

 cylindrical character, by thickening of the midrib and 

 loss of the lateral wings. 



The following observations are also to be made 



A. Examine the apices of young, actively-growing 

 branches : those branches which bear conceptacles 

 must be carefully avoided, as they do not show 

 such characteristic appearances : the extreme apex is 

 emarginate, or depressed, the base of the depression 

 being somewhat flattened, and marked by a_ slight 

 groove running in the plane of the thallus : it will be 

 seen subsequently that the initial cells lie at the 

 bottom of this groove. 



B. Compare a number of apices : in some only a 

 single emargination will be seen, in others two, 

 similar to one another, each having the groove at the 

 base, while others again will show an intermediate 

 appearance : from this it may be concluded that the 

 single apical point divides into two of equal strength, 

 each of which may develop into a branch of the 

 thallus similar to the original : thus the branching is 

 a dichotomy. 



