1 ^6 



EXTERNAL CONFORMATION OF PLANTS. 



these observations that they play no essential part in the whole external confor- 

 mation ; we have therefore only to do with the branchings which arise at the 

 end of a growing shoot, leaf, or root, or with terminal branchings. 



The terminal branches 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 shoots ; branch-systems of the first kind we will call simply 

 Dichoto??iies, of the second kind Monopodia. 



A Dichotomous Branch-system, according to the definition given in sect. 23, 

 is the result of the cessation of the growth at the apex in the direction previously 



taken, and its continuation in two diverging 

 directions at two newly constituted apical 

 points, which arise close beside the previous 

 one ; as is very clearly shown in Fig. 125 ^ 

 We will distinguish the two newly formed 

 branches by the term Bifurcations, the 

 member which produces them we will call 

 the Base of the bifurcation. From the 

 nature of the case every base can only bi- 

 furcate once ; but every branch may again 

 become the base of a new bifurcation ^. 



A Mofiopodiiim arises when the gene- 

 rating structure, following the direction of 

 its previous axis of growth, continues to 

 grow at its apex, while lateral structures 

 of the same name grow beneath it in 

 acropetal succession, the longitudinal axes 

 of which are placed in an oblique or transverse direction 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. 109, 113, 124). Every lateral branch may again 

 branch in the same manner, and thus itself become a monopodium of the second 

 order. Just as the dichotomy may consist of numerous bifurcations, so may a 

 monopodium consist of several orders of monopodial branching. 



These definitions refer only to the rudimentary condition of the branchings, 



Fig. 125.— Dichotomy of the thallus of Dictyota dkhotovia 

 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 

 w hich 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 the following section on Direction of Growth and Symmetry. 



2 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. 125; but when the segments are arranged 

 in two or three rows, this would necessitate that the dichotomising wall proceeding from the 

 apical surface of the apical cell should bisect its posterior 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 near it, were to change the direction of its longitudinal growth, so that thus 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 peculiarly well able to arrive at the 

 distinction between dichotomy and monopodium. Mutatis mutandis this is also true of Phanerogams 

 which have no apical cell. 



