7. BRANCH-SYSTEMS. 



1!) 



the branches all lie in one plane, and the roots of Selaginella r 

 where the branches lie in various planes : or the system may be- 

 come sympodial, if at each bifurcation one branch becomes more 

 strongly developed than the other; in such a case the bases of 

 the successive bifurcations appear to constitute an axis, which is 

 called the pseud-axis or sympodium, on which the weaker 

 branches appear as lateral branches (Fig. 9 B, Cf). The sym- 

 podium may consist of bifurcations belonging to the same side of 

 the successive dichotomies, either to the left or to the right (Fig. 

 9 -B), when it is said to be a hdicoid (bostrychoid) dichotomy, 

 e.g. the leaf of Adiantum pedatum : or it may consist alter- 

 nately of the right and left 

 bifurcations of successive di- 

 chotomies (Fig. 9 C), when it 

 is said to be a scorpioid (cin- 

 cinnal}, dichotomy, as in the 

 stem of most Selaginellas. 



2. The branching is said to 

 be racemose when the member 

 continues to grow in its ori- 

 ginal direction, and produces 

 lateral branches in acropetal 

 succession behind its apex ; it 

 is therefore the common base 

 of all the lateral shoots, and 

 hence the system is termed 

 monopodial. Each branch 

 may subsequently branch again 

 in the same manner. The pri- 

 mary axis continues to grow 

 more vigorously than the la- 

 teral axis, and each lateral 

 axis stands in the same rela- 

 tion to its lateral axes. 



3. The branching is said to 



be cymose, when at an early stage the growth of each lateral axis 

 begins to be more vigorous than that of the primary axis above 

 the point of origin of the lateral axis, and when the lateral axis 

 becomes more copiously branched than the primary axis. Hence 

 two forms may arise : 



(a) There may be no pseud-axis ; this is the case when two or 



FIG. 9. Diagram of the various modes of 

 development of adichotomous branch-system. 

 A One developed by repeated "bifurcation. 

 B Helicoid dichotomy; here the left-hand 

 branch is always more vigorous than the 

 right (r). C Scorpioid dichotomy; the right 

 and left branches are alternately more 

 vigorous in their growth. 



