38 VARIETIES OF BRANCHING [CH. 



very luxuriant growth— e.g. in Willows and Poplars, Elms, 

 &c. — and this is another common cause for non-sym- 

 metrical ramification. Of course the regularity is also 

 interfered with when accessory or extra buds are formed 

 side by side, or superposed, and develope into branches, 

 and when branches, already developed, are cast otf as 

 described on pp. 32 and 48. 



The student must not conclude that all branching is 

 brought about by the formation of lateral outgrowths 

 behind the apex, however, or that all the branches of the 

 shoot-system are necessarily axillary structures. 



It occurs rarely — chiefly in certain lower plants such 

 as vascular Cryptogams, Hepaticae and Algae — that the 

 vegetative cone stops its onward growth as a whole, and 

 proceeds to grow equally in two directions inclined at 

 some small angle to the original one : as if a knife edge 

 were held exactly on the apex, and growth proceeded 

 right and left of this. In these cases the stem becomes 

 forked, ami the branching is said to be dichotomous, but 

 true dichotomy probably never occurs in the higher 

 plants, where the branching is always lateral, however 

 deceptively like dichotomy the cases of the Lilac, Horse- 

 chestnut, Maples, &c. referred to above may appear. 



The branching of the shoots of many Ferns and lower 

 plants again is by no means always, or even usually 

 axillary, though it commonly starts from the leaf-bases : 

 in flowering plants however, apart from the examples 

 cited of adventitious branching and the development of 

 accessory buds, the rule is that one l)ranch ai-ises in the 

 axil of each of the many or few leaves concerned. 



An important cause of variety in the normal axillary 

 branching of the typical shoot depends on the relative 

 rapidity and duration of growth — i.e. elongation — of the 

 branch and its parent-axis. 



