THE GROWING POINT 



279 



form. This flattened appearance is generally developed after the opening of 

 the bud and the bud usually exhibits an exceedingly complicated arrangement 

 of its individual parts (prefoliation). 



A consideration of the branching of the stem and leaf should naturally 

 be followed by that of the branching of the root. There are, however, reasons 

 why a discussion of this subject 

 should be preceded by a study of 

 the cellular structure of the grow- 

 ing point. The varied methods 

 of formation of members which 

 we have hitherto studied occur 

 as we have already pointed out 

 (compare p. 273) not only in the 

 higher multicellular plants but 

 also in unicellular forms. This 

 is especially the case in the poly- 

 morphous genus Caulerpa (Siphon- 

 aceae), which in its habit resem- 

 bles one of the creeping forms of 

 the higher plants. Its dorsiventral 

 growing point produces a hori- 

 zontally growing stem, giving off 

 leaves from its upper side and 

 roots from below, and occasionally 

 forming lateral branches on the 

 sides, and yet the entire plant, 

 many centimetres or even deci- 

 metres in length, consists of a 

 single cell. The complete simi- 

 larity between the growing point 

 of this unicellular plant and that 

 of a multicellular type, proves most clearly that cell division cannot have the 

 importance which has for so long been ascribed to it. Nevertheless the subject 

 is naturally of interest in itself and we must devote a sentence or two to its 

 consideration. We shall put on one side the simple growing 

 points which go to form an unbranched plant-body (cell 

 filament, cell surface, or cell body) and glance only at the 

 branched forms, confining our attention further to those 

 which exhibit lateral branching. 



In the simplest case the growing point consists of 

 a single terminal cell, the apical cell. This cell (s in Fig. 75) 

 determines, for example, in Stypocaulon, the extension of the 

 chief axis, on which arise alternately to right and left the 

 lateral processes which are the rudiments of lateral branches. 

 They are scarcely formed before they are cut off by a con- 

 cavo-convex wall (/, Fig. 75) while the apex goes on growing. 

 When they have reached a considerable length they divide 

 into two cells by a transverse wall. The distal one retains the 

 characters of an apical cell, and will after a time develop 

 a lateral branch to the left, while the lower cell divides by successive walls into 

 a row of cells and thus becomes a cell body. 



This is an extreme case. Generally, the apical cell itself does not give 

 rise to lateral branches directly, but only indirectly, from the segments cut off 

 from it. The growing point is further not limited to the apical cell but includes 

 in addition a number of cells, and posteriorly merges gradually into cells which 



Fig. 64. Caulerpa proltfera. a, growing point ; 6, leaves ; 

 r, roots. From the Bonn Textbook. 



Fig- 75- Styfiocaulon 

 .\ropariunt. s, the apical 

 cell; /, 3, 1, 4, succes- 

 sively older lateral 

 branches. 



