GROUPINGS OF CELLS. 437 



groups or layers of cells (it may be with subsequent cell-divisioiis) take§ place. In 

 this case individual cells or groups or layers of cells may grow differently, their 

 volume and form altering in a different way from the remainder, and in this case ths 

 one must necessarily have a definite effect on the other : mechanical influences of 

 single cells or groups on those surrounding them^must take place; tensions and 

 pressures must make themselves felt, and only an exact investigation and con-- 

 sideration of the prevailing circumstances can give information as to what 

 mechanical processes have co-operated in influencing the arrangement of the 

 cells in the organ. We meet with this case wherever we are concerned with 

 the external and internal development, and therefore the later phases of growth 

 of the organs ^ 



(2) The processes are otherwise so long as an organ is still entirely in the 

 embryonic condition. In this case experience teaches that the entire mass of 

 embryonic tissue grows as a whole ^, th^t definite geometrical and mechanical 

 relations exist between the whole arrangement of cells and the outward form of 

 the growing organ. Only when the tissue passes from the embryonic condition 

 into the second phase of growth, does the grouping and differentiation in the 

 interior of the organ mentioned under (i) begin, and at the same time the pattern 

 of the cell arrangement becomes essentially different. 



(3) The fact that organs of like external form may nevertheless present quite 

 different patterns of cell net-works in their interior, causes peculiar difficulties in the 

 understanding of these processes. In other words, the distribution of the processes 

 of growth in the interior of organs which possess like external form may be very 

 different. 



In our further considerations we wiU confine ourselves, in the first place, to the 

 relations presented in the embryonic tissue, and further, to such cases where the 

 arrangement of the cells of the embryonic tissue are no longer essentially altered by 

 subsequent processes of growth, as the latter occurs especially in the development 

 of the woody mass from the cambium, and in the growth of various plants of simple 

 organisation. 



Fig. 273 represents the view from above of a flat extended Alga {Melobesia), 

 and for the sake of simplicity we may assume that the entire body of the Alga 

 consists of a simple layer of cells only (which, as a matter of fact, however, is not 

 the case). It is easily recognised that the portion of the disc, expanded to a certain 

 extent like a fan, has been produced by a further growth outwards having taken 

 place on the one side of the primitively elliptical germinal disc. The cell-network in 

 this case comes into existence by the flat extended plant-substance having become 

 divided into small areolae — i. e. cells — by two systems of lines, which correspond : to 

 the cell-walls. Of these systemis of lines, or directions of cell-walls, the one series 

 radiate in a fan-like manner towards the periphery of the body of the plant, which 



' I shall refer to this point more in detail in Lecture XXXIII. 



' Cf. note I, p. 431. With reference to a statement of PfefFer in his ' Pflanzen-physiologie^ 

 II. pp. 97, 98, I need only remark that my quoted treatise ' Vier Zellenanordnung und Wachsthum ' 

 was only possible from my regarding the cell-wall network as a, necessary consequence of 

 'Wacksthumsbewegungi' this had not been done before, apart from Hofmeister, with reference 

 to whose view I have remarked all that is necessary. 



