274 



METAMORPHOSIS 



The formation of branches at the growing point may take place in one or 

 other of two ways. In many plants two branches are formed at the same time ; 

 the previous direction of growth is abandoned, and instead of the prolongation 

 of the plant body in a straight line a forking or dichotomy appears. The alga 

 Dictyota may be cited as an example of this type. Here the dichotomy is 

 effected by the longitudinal segmentation of the apical cell (a in Fig. 63). 

 Similar cases of dichotomy are found in many liverworts ; in higher plants, 

 on the other hand, this method is seldom met with (shoots and roots of Lyco- 

 pqdium and Selaginella). In the highest plants generally, and also very fre- 

 quently in the lower plants, another type, viz. lateral branching occurs, that is 

 to say, in addition to the elongation of the body in a straight line, there are 

 formed also lateral projections on the growing point, so that we may distinguish 

 an axis and its lateral branches. As a rule, lateral branches are formed not once 

 only or one at a time on the growing point, but they occur for the most part 

 in great numbers continuously or at periodic intervals, and their developmental 

 succession is quite definite, in fact, progressive. The youngest are nearest to 

 the growing point, and the further we get from it the older do we find the 

 lateral branches to be. According as the growing point is situated at the fixed 



Fi{*. 64. Arthrocladia vfllosa, showing a basal 

 growing point v ; i, 2, 3, &c., indicate the basipetal 

 development of the lateral branches. After FALKEN- 

 BERG (1882, Art. Algae in SCHENK'S Handbuch der 

 Botanik). 



Fig. 65. Dasycladiisclavaeformis. 

 Growing point with three whorls of 

 branches. After CRAMER (1887) 

 (x 40). 



base or the free apex of the axis, the progressive development is basipetal or 

 acropetal. Fig. 64 shows an example of basipetal growth, while the more 

 ordinary acropetal development is illustrated at Figs. 65 and 66. 



Definite relations of symmetry, purely geometrical in character, exist in 

 the arrangement of the lateral organs at the growing point, which we must 

 study more carefully. We are able to distinguish radial, bilateral, and dorsi- 

 ventral growing points. In radial symmetry we find three or more longitudinal 

 regions exactly like each other, in bilateral symmetry there are only two such, in 

 dorsiventral symmetry only one plane. 



To take an example, Fig. 65 shows us a radially symmetrical growing point 

 of Dasycladus, a member of the Siphonaceae. This seaweed consists of one 

 long cell fixed at the base and elongating at its apex. The lateral appendages 

 are arranged in regular whorls on cask-like swellings of the axis. Immediately 

 below the apex of the growing point arises a whorl of fourteen branches, and 

 at a certain distance behind are two successively older whorls each composed 

 of the same number of branches, whose bases only are shown in the figure. 

 The successive whorls alternate (NoLL, 1896), that is to say, each of the branches 

 of the whorl above stands over the gap between each pair of branches of the 

 whorl next below. We find this alternation of branches to be the general rule 

 in whorled arrangements of lateral organs, whether we are dealing with simple 

 projections from a single cell, as in Dasycladus, or with complex organs such as 



