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DEVELOPMENT OF THE MEMBERS OF A BkANCH-SVSTEM. 



177 



ranches 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 af 

 the end of a growing shoot, leaf, or root, i. 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 Dichotomies, 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 2. 



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 obliquely 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 X^^tCAms Qi Dictyota dichotoma 

 (after Nageli) ; 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 

 mjitandis 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. 



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