524 SIGNIFICANCE OF CAMBIUM 



commonly occurs in the tangential diameter of the cells; but in the 

 gymnosperms the length of the initials is so great, in proportion to 

 their breadth, that the volume curve closely parallels that for the lon- 

 gitudinal dimension. Under normal conditions the volume of the 

 more or less isodiametric ray initials is very much less than that of 

 even the smallest elongated initials (Figs, 1 and 2) and is of the same 

 general order of magnitude as that of the undifferentiated cells of the 

 embryo and terminal meristems or growing points. 



To maintain a proper ratio between the two types of initials, in a 

 layer whose periphery is continually increasing in area, new aggrega- 

 tions of ray initials are periodically carved out of the large initials by 

 appropriate divisions of these elements (Text-fig. 3, D) , Furthermore, 

 under abnormal environmental conditions, e.g. injuries, all the elon- 

 gated initials in a given area may be induced to divide into small, 

 more or less isodiametric cells (Text-fig. 4, C and D). During the 

 subsequent growth of this region of the meristem, certain of these 

 small cells elongate and finally regenerate initials of normal dimen- 

 sions, as shown by dotted lines in Text-fig. 2, E and E'. 



The Problem of the Working Sphere of the Nucleus and the Maximum 

 Size of Undifferentiated Plant Cells. 



Sachs and Strasburger almost simultaneously called attention to 

 the fact that undifferentiated, actively dividing and growing cells of 

 plants, such as occur in embryonic and meristematic tissue, are rela- 

 tively minute, and concluded that this was undoubtedly due to the 

 fact that the working sphere of the nucleus is very restricted. Stras- 

 burger found that, in the case of "embryonic" cells of the growing 

 points of various plants, the ratio between the average diameters of 

 the nuclei and of the cells is as 0.003-0.016 mm. : 0.005-0.024 mm., or 

 as 2:3; and Sachs pointed out that, although plants vary enormously 

 in their linear dimensions (0.001 to 100,000 mm.), there is not a pro- 

 portional variation (0.001 to 0.05 mm.) in the size of their constitu- 

 ent cells. Both investigators emphasized the fact that, even in highly 

 differentiated tissues, unusually large or much elongated protoplasts^ 

 tend to be multinucleate. 



^ Such as have been critically studied by Schmitz, Treub, Kallen, Johow, 

 Haberlandt, Pirotta, Buscalioni, and many others. 



