432 



LECTURE XXVII. 



purely mechanical, considerations, which can only be rendered quite clear by very 

 careful thinking ; since, however, I have set myself the task in these lectures of only 

 expounding what is immediately capable of comprehension, I must confine myself to 

 making clear a series of the most elementary and easily intelligible facts. 



Above all, it is important to point out that, apart from a few exceptions, the 

 directions in which the new cell-walls of a growing plant-organ appear, depend upon 

 the internal distribution of growth as well as upon the external form of the growing 

 organ : the direction of any newly formed division-wall whatever is determined in 

 advance by these factors. Sections through growing, and especially through young 

 parts of the plant, always show arrangements of the cells which are quite definite, and 

 in the highest degree characteristic ; the directions of the cell-divisions are by no 

 means accidental, and an observer sufficiently acquainted with geometrical and 

 mechanical science at once recognises in the structures presented by the totality of 

 cell-walls within an organ, cut in the proper manner, that we have here to do with 

 a conformity to law, the true meaning of which, however, is difficult to decipher. 

 It will be well, therefore, in the first place to illustrate the dependence between 

 growth and cell-division by a few examples of the simplest kind. 



The simplest case is presented by thin, filamentous organs, which consist of a 

 single row of cells, as in the case of many Algae and in the hypise of Fungi. As a 

 rule, the cell- walls here occur as transverse septa of the filament — i.e. each new 

 division-wall cuts the long axis and the circumference of the filament at right angles. 

 Nevertheless a few exceptions are found even here. In the .root-filaments of the 

 true Mosses, as well as of the Characese, the transverse septa are oblique to the 

 long axis, for which in the meantime no satisfactory explanation can be given. 

 However, in contrast to the enormously large number of cases in which cell- 

 filaments are divided by transverse walls cutting at right angles, these are but rare 

 exceptions. 



Wh«n we take into consideration the growth and cell-division of isolated cells the 

 volume of which becomes enlarged in all directions, the problem is a more com- 

 plicated one. We meet with such, for instance, in a large number of simple Algae, 



Naegeli aptly remarks, like the three cleavage planes of a crystal. By means of stratification and 

 stiiation the substance of a cell-wall is cut up into polyhedral areolae, so that the three systems of 

 densest layers form a network, in the meshes of which the less dense areolse (containing most water) 

 are enclosed. The substance of a thick cell-wall is built up like the primary meristem of a growing- 

 point. The cell-walls mutually cutting in th;-ee directions correspond to the densest lamella of 

 a thick cell-wall, and the protoplasmic bodies of the cells of the primary meristem to the soft areolae. 

 I will not here foUow this comparison (which is without constraint) further, but only remark that 

 it becomes the more apt the smaller the cells of the primary meristem are.' 



Two years after this statement of mine a treatise by Schwendener appeared, ' Ober die iurch 

 Wachsthum bedingte Verschiebung kleinster Theilchen in trajektorischen Curven' (Monatsber. derkgl. 

 Akad. d. Wiss. zu Berlin, 1880), where he entirely adopts the new points of view which I had opened 

 out ; only that (p. 426) he believes that he finds an important difference between his own view and mine. 

 He says, ' According to my (Schwendener's) view the cell-divisions constitute an independent pheno- 

 menon — and the formation of series of trajectories is everywhere ruled' by the same mechanical prin- 

 ciples which condition the direction of the rows of micellae in starch-grains and thick cell-walls,' &c. 



I consider that the quotation from my treatise, which had appeared two years previously, differs' 

 from Schwendener's view just cited only in so far that it puts exactly the same ideas more in detail 

 and more clearly, and at the same time directly contradicts Schwendener's older views (1877) on the 

 relation of growth and cell-division. 



