THE GROWING POINT 



281 



each individual anticlinal cell row has become twice as broad as it was, and has 

 become divided by new anticlinal walls into two layers of cells. The periclinal 

 rows have elongated considerably, but have become obviously broader at their 

 bases only, where further divisions have also taken place. This result has been 

 arrived at on the special assumption that all the anticlines, 7-11, have been 

 growing at about the same rate, and this assumption may be true in nature of 

 a certain part of the growing point. It cannot, however, be generally true ; 

 for a maximum or a minimum growth may equally well occur at the apex while 

 towards the base a gradual change ensues. Since, unfortunately, the anticlinal 

 rows are not so obvious in nature as in the diagram, we are on the whole ignorant 

 of the exact nature of the growth divisions taking place in them. We know 

 more about the periclinal divisions ; their gradually increasing transverse exten- 

 sion may be deduced off-hand from Fig. 76. In the illustration the growth 

 division has not as yet proceeded so far as to result in periclinal division. If 

 the growing point be less conical, periclinal divisions soon appear beneath the 

 apex. It is only on the outermost layer, the future epidermis (d, Fig. 76), 

 that these periclinal divisions cease in all cases. In the growing points of 

 mosses and ferns the distribution of intensity of growth can be made out much 



Fig. 78. Diagram of a growing 

 point in longitudinal section. The 

 right hand half represents a later 

 stage in the development of the left 

 hand half. 



K 



Fig. 79. Diagram of the growing point A^ 

 with successively older leaf rudiments, B^ C, D. 

 After SACHS (1879). 



more accurately than in Phanerogams owing to the very regular divisions 

 resulting from the activity of the apical cell (compare WESTERMAIER, 1881). 



The rule followed at the growing point as to the direction of the new cell- 

 walls is that which we have already become acquainted with. The new walls 

 appear throughout as ' minimum areas ', and in very many cases the new walls 

 arise at right angles to those already in existence. We cannot follow out this 

 subject further however. 



Leaf formation always occurs below the apex of the growing point. In 

 the mosses this is seen especially clearly because in that group each segment 

 cell gives rise to a leaf. The leaves are thus laid down in a one-third spiral, but 

 the segment cells of the axis which give rise to them at once undergo peculiar 

 changes in form (CoRRENS, 1899; SECKT, 1901), which bring about torsions and 

 so lead to great complexity in the leaf arrangement. The outer wall of the seg- 

 ment cell forming a leaf rudiment bulges outwards, and from this bulging is 

 next developed the two-sided apical cell destined to give rise to the leaf. In 

 the ferns relationships as intimate as these between the stem segments and the 

 leaves can no longer be distinguished, and the origin of the latter takes place 

 as in the Phanerogams, save that in the ferns the apical cell of a leaf may still 

 be distinguished, while in the Phanerogams it cannot. The leaf formation of the 

 Phanerogams may be made out from Fig. 76, but more clearly, perhaps, from 

 SACHS'S diagram (Fig. 79). In the diagram three successively older leaves B, 

 C, D, are represented beneath the apex A. It shows that the leaf arises from 



