36 PART I. THE MORPHOLOGY OF PLANTS. [ 7. 



two highest lateral buds carry on the development of the branch ; 

 and in those of HJiamnus cathartica, the terminal bud of which 

 becomes a thorn. 



(/?) A pseud-axis, or sympodium, is formed when only one lateral 

 axis developes vigorously in each case. Thus, in Fig. 19 A. in 

 \vhich the darker lines indicate the more vigorous growth, the 

 lateral axis has grown more vigorously than the parent axis, and 

 so on. The pseud-axis, which is thus formed, is at first crooked; 

 but in most cases it subsequently becomes straight (Fig. 19 A 

 becomes Z). Examples of the formation of sympodia are 

 afforded by many subterranean stems, such as that of Polygo- 

 natum (see p. 42, Fig. 23 B), which annually come above ground r 

 whilst a lateral branch maintains the direction of growth under- 

 ground. Much the same thing may be observed, though not so- 

 clearly, in the branches of many shrubs and trees. Amongst 

 inflorescences, also many examples of the formation of sympodia 

 are to be found (see Part III.). The vigorous lateral branch 

 which carries on the further growth of the plant is termed an 

 innovation- shoot. 



The various forms of sympodial branch-systems maybe classified 

 as follows: 



1. Sympodia in which all the branches lie in one plane, forming 

 a bilateral branch-system : 



a. The Rhipidium (fan) ; the lateral branches are developed! 



alternately in two opposite directions (Fig. 19 A, J3). 



b. The Drepanium (sickle) ; the more vigorous branches are- 



all developed on one side (Fig. 19 D). 



2. Sympodia in which the branches lie in different planes,, 

 forming a radial branch- system : 



a. The Scorpioid Cyme (Cincinnus) ; the lateral branches 



are developed alternately on opposite sides. 



b. The Helicoid Cyme (Bostryx) ; the lateral branches are 



all developed on one side. 



7. Cohesion and Adhesion. It sometimes happens that 

 the originally free edges of parts subsequently grow together; 

 for instance, the margins of the carpellary leaves to form ovaries. 

 In some Florideos, originally separate branches of the shoot have 

 'been observed to grow together. But a more common case is that 

 the rudiments of distinct members become united into one whole 

 by the growth of their common base (see p. 17). For example, a 

 gamopetalous corolla (see infra, Part III.) arises in this way, that 



