THE NUCLEUS 63 



terminal meristems of plants such a rule may well hold true within 

 limits, but the condition reported by Bailey (1920) in the lateral meristem 

 (cambium) shows clearly that it cannot have universal application. 

 The cambial initials may vary enormously in size with no corresponding 

 variation in the size of their nuclei: two such initials, one of them having 

 many hundreds of times the volume of the other, may possess nuclei 

 of approximately equal size. 



The nucleoplasmic ratio has figured prominently in discussions of 

 the problem of senescence. R. Hertwig in 1889 advanced the theory 

 that senescence and natural death are associated with an increase in the 

 relative size of the nucleus. He later asserted (1903, 1908) that the nucleo- 

 plasmic relation is self-regulatory within certain limits for each kind of 

 cell, exercising thereby a control over many cell activities, including cell- 

 division. Minot (1891, 1908, 1913), on the contrary, believed that the 

 increase in the relative volume of the cytoplasm, in addition to its differ- 

 entiation, is a fundamental factor in senescence and death. Conklin 

 (1912), as a result of his work on Crepidula, denied the existence of a 

 constant and self-regulatory nucleoplasmic relation, holding rather that 

 changes in this relation are not causes of such cell activities as cell- 

 division, but are results of the metabolic processes by which such cell 

 activities are brought about. Child (1915) points out that in most 

 animal tissues there is an increase in the relative amount of cytoplasm 

 during senescence, whereas in plants, although the cell enlarges through 

 vacuolation, the relative volume of cytoplasm often does not increase. 

 He therefore concludes that the nucleoplasmic relation cannot be regarded 

 as a universal factor in senescence ; it is rather an indication of the kind 

 and rate of metabolism. The differentiation of the cytoplasm, apart 

 from its mere change in volume, Child, with many other workers (Minot. 

 Delage, Jennings, etc.), regards as a matter of the greatest importance in 

 senescence. Further discussion of this subject is deferred to Chapter VII. 



Not only has it been held that there is a certain relation between the 

 mass of the nucleus and that of the cytoplasm, whatever the significance 

 of this relation may be, but there also seems to be a size relationship 

 between the nucleus and its contained chromosomes. In 1896 Boveri 

 showed that the size of the nuclei in merogonic echinoderm larvae (see 

 p. 325) is dependent upon the number of chromosomes each contains. 

 In a more extended study (1905) he demonstrated that it is the surface 

 of the nucleus that is proportional to the chromosome number, and also 

 that the size of the cell is proportional to both. Gates (1909), however, 

 adduced evidence to show that this rule is by no means universal. 



Structure. Having reviewed the general features of the nucleus as 

 a whole, we may next give attention to its structure, as seen in typical 

 cases. 



The nucleus is bounded by a distinct nuclear membrane. The nature 



