454 



LECTURE XXVII. 



septa are placed obliquely. The significance of the apical cell comes out more 

 conspicuously and characteristically when the products of division (segments) arising 

 from it not only themselves grow further but also undergo essential changes of form 

 and corresponding cell-divisions, as is the case in many Algse, and especially in the 

 Characeae : Fig. 290 may serve as a scheme for the latter. The cell marked v, at the 

 end of the shoot, is in this case the apical cell, from which all the organs and tissue- 

 cells of this plant can be derived by the formation of segments and the growth 

 and further division of the latter. Every time this cell elongates a little in the 

 direction of the longitudinal axis, there appears in it a transverse wall, an anticline, 

 convex downwards. The cell cut off by this is the segment. This now grows also 

 in length and in circumference, and after a short time is divided into two cells 

 by means of a transverse wall convex upwards, viz. into an upper one which possesses 

 the form of a bi-concave lens and into a lower one shaped like a bi-convex lens. 

 The growth and further fate of these two daughter-cells of the segment in this case 

 then differ in important respects. The lower bi-convex cell thenceforward grows 

 vigorously in length, without undergoing further divisions; the figure shows how 

 this cell gradually assumes the forms z", i", t'", i"" , and how the longitudinal axis 

 of the shoot is produced by numerous such cells. The bi-concave daughter-cell 

 of the system, on the contrary, soon undergoes vertical divisions, and is trans- 

 formed into a disc, or so-called node, consisting of cells, since it grows chiefly 

 in the transverse direction, and only very little in the longitudinal direction of the 

 shoot. Certain cells situated at the margin of this disc now grow out and project 

 upwards, b' ; these outgrowths then develope further into the leaves V , V" , V" , 

 and subsequently, by the growing out of certain cells at the nodes, lateral shoots 

 K, and from the leaves themselves sexual organs a and arise. It will best be 

 seen what can be produced from a segment of the apical cell by means of the 

 growth described and the subsequent cell-divisions, on observing that the parts 

 marked in each case alike with i and b and surrounded by a thick contour have 

 always proceeded from one segment. It would occupy too much time for our 

 purpose to show how all the various cells in the tissues of these organs gradually 

 arise from the originally bi-concave nodal cell, in perfectly definite sequence and 

 in accordance with the law of cell-division described above, and how these organs 

 eventually become altered in form by means of elongation and increase considerably 

 in size. 



On account of its simplicity and intelligibility. Fig. 291 may be examined in 

 addition with respect to the apical cell and its segmentation. This represents the 

 growing-point of an Alga, Didyoia, which has just split into two similar growing-points, 

 a and b, by so-called dichotomy. In a we see the apical cell, shaped in accordance 

 with the slightly arched form of the thin ribbon-like shoot, like a bi-convex lens, 

 from which, after having elongated a little in the direction of the axis of the shoot, 

 the segment i has been produced, and this has already become divided by a median 

 longitudinal wall into two equal halves. The groups of cells marked 2, 3, 4, and 5, 

 are older segments of the apical cell, in which, as they have grown in length as well 

 as in breadth, more and more numerous anticlinal and periclinal walls have gradually 

 made their appearance ; and here also it is easily understood how the whole mass of 

 tissue of the shoot arises from the segments of the. apical cell. In b, a longitudinal 



