NOTES 7 1 :; 



32. Sachs : Flora, 77, 1893 (growth periods and formative stimuli). Noll : 

 Biol. Centr. (embryonic substance). 



33. Krabbe : Das gleitende Wachstum, Berlin, JS8ti. Jost: 15. Z. I'.HH (cam- 

 bium). Von Guttenberg : Sitzb. Wien. Ill, 1902 (crystal-cells of < iitrus). 



34. At the time of publication of the second [German] edition of this book, 

 the author agreed with Zimmermann, in considering it improbable that neighbouring 

 cells should become displaced relatively to one another by means of sliding growth. 

 It seemed sufficient to assume, that when growing cells pushed their pointed ends 

 or sharp edges between other cells, local surface-extension took place, which only 

 affected the overlapping portions of the cell- membranes. More careful consideration, 

 however, shows that this hypothesis is in reality far more open to criticism than 

 Krabbe's theory of sliding growth. In order that the pointed end or sharp edge 

 of a cell may insert itself between two adjoining cells, without any sliding of the 

 cells over one another, the surface-extension must be strictly confined to the actual 

 tip or edge. Surface growth must cease absolutely, close behind the tip, or edge 

 so that the fresh layers of cell- wall-substance may be laid down in their final position 

 upon the extended older parts of the membrane, otherwise sliding growth is bound 

 to occur. It is, however, extremely improbable that surface-extension is always 

 or even commonly so strictly localised. 



The strongest argument against the theory of sliding growth consists in the 

 universal occurrence of protoplasmic connections between adjacent cells. But there 

 seems no reason to doubt that these connections may be formed secondarily, i.e. 

 after sliding growth has come to an end. According to Strasburger, the connecting 

 threads traversing the walls of latex-tubes are invariably secondary in origin. Of. 

 Strasburger : P.J. 36, pp. 506 sr^. Jost : I.e. [33], pp. 8 and 10. 



35. Nageli : Die neueren Algensysteme, Zurich, 1847. Id. Beitr. 1, 1858. 



36. Nageli and Schwendener : Das Mikroskop, 2nd ed. 1877, pp. 554-568. 



37. According to the view embodied in the text, an apical cell represents a 

 persistent initial cell. This conception of the apical cell is, in the author's opinion, 

 the only reasonable one. Another view, which has received strong support in certain 

 quarters, denies the existence of a single persistent rhythmically dividing apical 

 initial ; at each division the apical cell is supposed to give rise to two daughter- 

 cells, one of which represents a primary segment, while the other takes the place 

 of the self-abolished apical initial. According to this theory, therefore, there is a 

 constant succession of apical initials, each surviving only long enough to cut off 

 a single primary segment. This second interpretation, however, amounts to little 

 more than a play upon current phrases. It is customary, namely, to designate the 

 two cells formed at each act of cell-division, daughter- eel Is with reference to their 

 mother-cell, and sister-cells with reference to one another. This metaphorical 

 terminology for it is nothing more is only appropriate when the two daughter- 

 or sister-cells agree with one another morphologically and physiologically. But 

 this condition is far from being fulfilled as regards the products of the division of 

 an ordinary apical cell. At every division, one of the products, the segment, is a 

 new formation, both from the morphological and from the physiological point of 

 view ; the other retains all the features of the original apical cell unchanged. The 

 only proper usage, in the present instance, therefore, is to describe the apical initial 

 as a mother-cell, which cuts off the primary segments as daughter-cells ; all these 

 segments are sister-cells with reference to one another, but none of them ought to 

 be termed a sister-cell of the apical initial, which is their common mother-cell. The 

 conception of a persistent apical cell is thus seen to be a perfectly legitimate one 



