170 MORPHOLOGY OF MEMBERS. 



Dichotomous branching is very common among Thallophytes, especially Algae 

 and the lower Hepatica^ ; among Phanerogams it occurs only exceptionally ; among 

 Vascular Cryptogams it appears to occur in Ferns {e.g. the leaves of Platy cerium 

 alcicorne) ; but it is the only mode of branching in all shoots and roots of Sela- 

 ginelleae, Lycopodieae, and in the roots of Isoeteae ^ 



(3) The origin of lateral members, whether similar or dissimilar to the pro- 

 ducing member, is either exogenous or endogenous. The former term is applied 

 when they are formed by lateral outgrowth of a superficial cell or of a mass of 

 cells which includes the outer layers of tissue, as in the case of all leaves and 

 hairs and most normal leaf-bearing shoots. A member is of endogenous origin 

 when it is covered, even when in a rudimentary condition, by a layer of the tissue 

 of the producing member which does not take part in the new formation, as in 

 all roots, all lateral shoots of Equisetaceae, and in adventitious buds. 



(4) Lateral members of any kind are almost always formed in considerable 

 numbers on the axial structure which produces them, and even repeatedly one 

 after another, because the producing structure continues to increase in length, 

 and the conditions for similar equivalent outgrowths are repeated. Thus the 

 stem, so long as it continues to grow at the apex, produces leaves, hairs, often 

 even roots, and generally lateral shoots in great numbers, one after another; 

 roots usually form in succession many lateral roots, branching leaves usually several 

 segments. If the apical growth ceases early, the number of the lateral members is 

 also limited; thus the short primary stem of Welwitschia mirabilis produces only 

 two leaves. When the increase in length of the stem is very slow, the formation of 

 lateral shoots from it is sometimes altogether suppressed, as in Isoeies, Boirychhim, 

 and Ophioglossum. 



(5) An axial structure may produce either several equivalent lateral members 

 at the same level, or only one; in the second case the members formed in 

 succession are termed solitary, in the first case a Whorl or Verticil. Leaves 

 often occur in whorls, branches less frequently, roots occasionally (in the primary 

 roots of Phanerogams). In the same whorl the members may arise either simul- 

 taneously, as the petals and stamens of many flowers, or the foliage-leaves of 

 many Phanerogams ; or successively, as in Characese and Salvinieae. A whorl is 

 a true one when the zone on which the lateral members are inserted is trans- 

 verse from the first, as occurs in both the last-named plants and in many flowers; 

 Spurious Whorls, on the other hand, are formed by displacement and unequal 

 growth in the axis, as in Equisetaceae, where the leaves, roots, and branches arise 

 from transverse zones which are themselves formed by displacement of three segments 

 of the stem '-. 



(6) Similar and equivalent lateral members usually arise on their common 

 axial structure in acropetal or basifugal order, i.e. the younger a member is the 



^ For further details of lateral branching and dichotomy see the conclusion of this section and 

 Sect. 25. 



^ The three segments, which together form the periphery of the stem, stand at first at different 

 heights, but arrange themselves, as Rees has shown, in a transverse zone, which developes ex- 

 ternally a circular protuberance, the rudiment of the leaves (see Book II, Equisetacese). 



