DEVELOPMENT OF THE MEMBERS OF A BRANCH-SYSTEM. 1 77 
branches of the various orders. We may here leave adventitious branchings en- 
tirely out of consideration; for it is evident that in respect to the phenomena 
now under consideration they play no essential part in the building up of the 
whole plant. We have therefore to do only with the branchings which arise at 
the end of a growing shoot, leaf, or root, t. e. with terminal branchings. These 
may be referred (as has already been shown in Sect. 24, div. 2) to two principal 
forms, dependent on the origin of the branching by dichotomy or by lateral 
branching; branch-systems of the first kind may be called simply Dichoiomies, of 
the second kind Monopodia. 
A Dichotomous Branch-system, according to the definition given in Sect. 24, is 
the result of the cessation of the growth at the apex in the original direction, 
and its continuation in two new direc- 
tions at newly constituted apical points, 
as is very clearly shown in Fig. 133 \ 
We may term the newly formed branches 
Bifurcations, and the member which pro- 
duces them the Base of the bifurcation. 
Every base can only bifurcate once; but 
every branch may again become the base 
of a new bifurcation ^. 
A Monopodium arises when the gener- 
ating structure, following the direction of 
its previous growth, continues to grow 
at its apex, while lateral structures of a 
like kind are produced beneath it in acro- 
petal succession, their longitudinal axes 
being placed obUquely or transversely to 
that of the generating member. The 
generating member, since it continues to grow during the branching, may form 
numerous lateral members; for all these it is the common base; hence the name 
Monopodium (Figs. 119, 123, 132). Every lateral branch may again branch in 
the same manner, and thus itself become a monopodium of the second order. 
Fig. 133.— Dichotomy of the thallus of Dicfyota dic/iotovia 
(after Näjfeli) ; the order of development is according to the 
letters A—E; the letters t—z indicate the segmentations of 
the apical cell before it dichotomises ; i is the division-wall by 
which the dichotomy commences ; 2 — 6 the segments of the 
new apical cells. 
^ Since we have to give here a narrower application to the term Direction of Growth, it will 
be necessary to compare with this Sect. 27. 
^ In Cryptogams with apical cells it may be thought that dichotomy must necessarily be brought 
about by longitudinal division of the apical cell. When the segments arise by transverse division 
this is actually the case, as is shown in Fig. 133 ; but when the segmentation of the apical cell takes 
place in two or three rows, this would necessitate that the dichotomising wall should bisect its 
inferior angle, and thus have a position which is apparently universally avoided in cell-division. 
It is nevertheless possible that a true dichotomy may take place without this. Suppose the old 
apical cell, immediately after the formation of a new one by its side, were to change the direction 
of its longitudinal growth, so that both apices diverge from the previous direction of growth ; the 
old apical cell then represents the apex of a new direction of growth. From this it seems to me 
that we are able to arrive at the distinction between a dichotomy and monopodium. Mutatis 
mutandis this is also true of Phanerogams which have no apical cell. It is necessary here again 
to point out that the occurrence of transitional forms between dichotomies and monopodia does 
not prevent our giving an exact definition of these terms ; it is only, in fact, by this very means that 
transitional forms can be recognised as such. 
N 
