i8 



MORPHOLOGY OF TISSUES. 



called the Apical Cell. In some Cryptogams, on the other hand, and in Phanero- 

 gams, there is no single apical cell of this character. Even when a cell lies at 

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

 what is of greater 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 Piincium Vegetaiionis with and without 

 an Apical Cell. 



(a) Punctum Vegetaiionis with an Apical Cell. The formation of the primary 

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



in different ways, but it generally results from the re- 

 peated rhythmical division of each apical cell into two 

 unequal daughter-cells. One of the two daughter- 

 cells remains from the first similar to the mother- 

 cell (the apical cell), and includes the apex; it is 

 immediately enlarged by growth till it equals the pre- 

 vious 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, 

 on its part, remain undivided ; and then the whole 

 tissue which is produced from the apical cell appears 

 in the form of a simple thread or row of cells, as 

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

 rally the segment is also again divided into two cells, 

 each of which again breaks up into two, and this pro- 

 cess is mostly repeated many times in the daughter- 

 cells, until a more or less extensive mass of tissue is 

 FIG. 9?.-A branch of the thaiiome of produccd from thc scgmcnt. The primary meristem 

 fSX:Sd^^d[;::mS;ThiM^S! now consists of such portions of tissue. A very simple 

 ceiiwliif''^'''"* ''"''' '""""'"'' case of this kind is shown in Fig. 98, where the 



* The portions of wall which enclose a segment-cell are different 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. \Miere the segments arise as transverse discs of an apical 

 cell, the process is very complicated, from the segmentation taking place on two or three sides ; the 

 segments have in this case also side-walls as well as the two principal walls and the outer wall, 

 which intersect at oblique angles within and below. The side-walls are portions of the principal 

 walls of older adjoining segments, which are always bounded by the youngest partition-wall of the 

 apical cell, and this is at the same time the youngest principal wall. 



