530 SIGNIFICANCE OF CAMBIUM 



Dynamics of Cytokinesis. 



The phenomena of karyokinesis and cytokinesis in the cambium 

 are in direct contradiction to most generalizations concerning cell 

 division. According to Sachs' Law, successive division planes should 

 intersect at right angles; but in the cambium the successive longitu- 

 dinal partitions are parallel. Hertwig's development of Sachs' Law 

 hypothecates that the axis of the mitotic figures typically lies in the 

 longest axis of the protoplasmic mass, and division therefore tends to 

 cut this axis at a right angle; but in the cambium the axis of the 

 mitotic figure is usually placed either at right angles to the long axis 

 of the cell (angiosperms) or in a diagonal position (conifers, angio- 

 sperms). Errera's (Plateau's) Law of Minimal Area is based upon 

 the assumption that the recently formed membranes of cells are 

 semiliquid films which "tend to assume a form which would be 

 assumed, under similar conditions, by a liquid film destitute of 

 weight;" but in the cambium the partitions are commonly division 

 planes of maximal area. Furthermore, according to this law, the 

 division membranes of most plant cells'^ should intersect the sides of the 

 cells at right angles. De Wildeman, Thompson, and others contend 

 that even the occurrence of oblique divisions in elongated plant cells 

 does not necessarily invalidate the minimal area hypotheses, pro- 

 vided these partitions are sigmoid and intersect the side walls at 

 right angles. 



It is significant, accordingly, that in the cambial initials — as also 

 in their daughter cells — of gymnosperms and dicotyledons, many 

 partitions are formed which intersect the older, rigid walls at angles 

 of varying degrees of acuteness (Text-figs. 3 and 4). These acute 

 angles are not due to sliding growth and displacement of proto- 

 plasts. The peculiar type of cell plate formation in the cambium 

 facilitates the study of successive stages in the formation of the divi- 

 sion membranes and it is evident that the kinoplasmasomes, cell 

 plates, and new cell walls frequently intersect the sides of the cells at 

 very acute angles (Text-fig. 3). 



^ The older walls of most cells of the higher plants are relatively thick and 

 rigid. 



