840 A TEXTBOOK OF THEORETICAL BOTANY 



for particular biological purposes, whereby certain branches of limited 

 growth mav be modified in structure to a degree that is rarely possible with 

 the monopodial system. We shall deal with a number of such biologically 

 specialized branches at the end of this chapter, when we shall see that the 

 majority are the products of sympodial branching. To put it less teleologically 

 we may say that the biological specialization of a branch in most cases limits 

 its growth and makes sympodial branching inevitable. 



The distinction between " long " and " short " shoots, that is, between 

 those which are indefinite or unlimited and those which are definite or 

 limited in growth, is thus usually a question of the specialization of the 

 short shoot for some other purpose than that of growth, but the distinction 

 is not always sharp or fixed and transitions and reversions occur. An 

 important case is that of the reproductive shoots. Specialization for repro- 

 duction is usually associated with limited growth, and in the case of the 

 flower this specialization has gone so far that it is rare indeed for the axis 

 of a flower to resume indefinite growth. When it does so, the flower is said 

 to be " proliferating." 



Even those branches which are nominally unlimited in growth may 

 differ in their rates of growth. Most frequently, in woody plants such as trees, 

 the buds near the top of last year's growth form the longest branches, and 

 the length of branch produced by each bud decreases regularly dow^nward 

 to the base of that section of the shoot. The lowest branches on each year's 

 growth may remain permanently dwarf and form the reproductive shoots, 

 i.e., the flowering and fruiting " spurs," which are so important in fruit 

 trees (Fig. 834). In most cases, e.g., Apple and Pear, the growth of these 

 spurs is sympodial, but in the Plum it is monopodial. 



The angles at which branches are borne are very variable between species, 

 but are relatively constant within the species. Spreading branches are 

 described as patent, those which lie close to the main axis are called 

 fastigiate, and an alteration in this character may mark a distinctive variety, 

 as in the fastigiate Lombardy Poplar [Popidus nigra, var. italica) in which the 

 branches all tend to be vertical. 



Branches which make a wide angle with the vertical, i.e., plagiotropic 

 branches, frequently assume a dorsiventral symmetry through the horizontal 

 placing of leaves and of side shoots, or by a marked difl^erence in the size of 

 the leaves on the upper and lower sides of the branch respectively. Although 

 this has a direct physiological meaning, in that it ensures the best exposure 

 of the leaves to light, yet this symmetrical character is often permanently 

 fixed in the organization of the branch. If such plagiotropic branches are 

 removed and used as cuttings for propagation, the dorsiventral habit is often 

 retained and a plagiotropic plant is the result. 



Branches are classified under different orders. Thus the primary vegeta- 

 tive axis and all equivalent branches from it belong to the first order ; 

 flowering shoots belong to the second order, and the flowers themselves to a 

 third order. If the primary axis is an underground structure then the leafy 

 shoots above ground become branches of the second order and so on. Four 



