THE PRIMARY ME RI STEM AND THE APICAL CELL. 



139 



apex, it is not, as in the former case, distinguished by its greater size ; and, what 

 is of more importance, it cannot be recognised as the single original mother- 

 cell of all the cells of the primary meristem, nor even of a definite layer. We 

 may distinguish, therefore, between the Growing Point with and without an Apical 

 Cell. 



(a) Growing Point with an Apical Cell. The formation of the primary 

 meristem out of the apical cell may be brought about, as will be shown presently, 

 in different ways, but it generally results from the 

 constant repeated division of the apical cell into two 

 unequal daughter-cells. One of the two daughter-cells 

 (the Apical Cell) remains from the first similar to the 

 mother-cell, and includes the apex; it is immediately 

 enlarged by growth till it equals the previous apical 

 cell in size, and then again divides, and so on. This 

 process produces the appearance as if the apical cell 

 always remained intact ; and this has been assumed in 

 ordinary language, although the apical cell existing at 

 any time is only a daughter-cell of the preceding one. 

 The other daughter-cell, on the other hand, appears 

 from the first like a piece cut off from the back or side 

 of the apical cell, generally in the form of a disc or 

 angular plate, and is hence called the Segment^. In 

 the simplest case the segment may remain undivided; 

 and then the whole tissue which is produced from the 

 apical cell has the form of a simple row of cells, as in 

 some Algae, and in Fungus-hyphae and hairs. But 

 generally the segment divides into two cells, each of 

 which again breaks up into two, and this process is 

 mostly repeated many times in the daughter-cells, until 

 a more or less extensive mass of tissue is produced from 

 the segment. The aggregate of such masses of tissue 

 constitutes the primary meristem. A very simple case of 

 this kind is shown in Fig. 108, where the apical cell (j), 

 here very large, growing straight out from its base, is 

 divided by septa (/", /*), and thus forms the segments 

 which lie in a row one over another. But each of these 



Fig. 108.— a branch of the thallome of 

 Stypocaulon scoparium with two branchlets 

 X and y, and the rudiment of a third branch- 

 let z (after Geyler) ; all the" lines indicate 

 cell-walls. 



^ The portions of wall which enclose a segment-cell differ in their nature and origin, and 

 behave differently in their subsequent growth. Each segment possesses two walls which were 

 originally division-walls of the apical cell ; they are generally parallel to one another, and are called 

 the 'Principal walls' of the segment; the older faces the base, the younger the apex of the organ. 

 Another portion of the wall of the segment is a part of the outer wall of the apical cell ; it may be 

 termed the 'Outer wall' of the segment. Where the segments arise as transverse discs of an apical 

 cell, as in Stypocaulon and Characese, the outer wall is cylindrical ; when the segmentation takes 

 place on two or three sides the process is very complicated; the segments have in this case, 

 besides the two principal walls and the outer wall, side-walls which intersect at an acute angle. 

 The side-walls are portions of the principal walls of older adjoining segments, and are successively 

 cut off by the youngest septum of the apical cell, which is at the same time the youngest 

 principal wall. 



