THE PRIMARY MERISTEM AND THE APICAL CELL. 
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 mi 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-hyphse 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 {s)^ 
here very large, growing straight out from its base, is 
divided by septa (/", P), 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 scopariinn with two branchlets 
X and y, and the rudiment of a third branch- 
let ä (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 Siypocmdon and Characeae, 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 v/all. 
