THE CAMBIUM AND ITS DERIVATIVE TISSUES 
429 
early prophases (figs. 6, 41) and late telophases. Many of my preparations 
suggest that the large nucleoli are composed of aggregations of smaller 
nucleolar masses (fig. 43). During the process of aggregation hollow spheres 
(fig. 44) may be formed which resemble "vacuolated" nucleoli. The larger 
nucleoli also appear to fuse in many cases (figs, i, 33, 41), particularly in cells 
which are differentiating into tracheary elements or sieve tubes (fig. 35a). 
If nucleoli actually are concerned in the growth or enlargement of 
chromosomes, as is maintained by various botanists, one might expect the 
nuclei of fusiform initials (fig. 10), which contain a relatively large volume 
of nucleolar matter, to form much larger chromosomes than the small 
nuclei of ray initials (fig. 14). This is not the case, however, in Pinus 
Strohus, as I have shown on preceding pages. 
There is a striking contrast between the cambial nucleoli of gymno- 
sperms and those of dicotyledons. The latter tend to be surrounded by a 
conspicuous halo (fig. 40), which is absent in the Coniferae (Pis. XXVI, 
XXVII). Of course, this halo is supposed to be an artifact (Nageli), yet 
the fact that it occurs so constantly in one group and not in the other sug- 
gests that it may be significant from the morphological or the physico- 
chemical point of view. 
Schwarz (1884) and Zacharias (1895) noted that during the differentia- 
tion of tissue elements in the growing points of plants, the volume of the 
nucleoli, as of the nuclei, first increases and then decreases. Similar phe- 
nomena occur in the cambial zone of Pinus Strohus. During the early 
stages of the differentiation of tracheids and sieve tubes, the nuclei become 
coarsely granular. The nucleolar mass subsequently decreases (figs. 35, 
36, 37), and the nucleus becomes profoundly modified in shape and loses its 
staining capacity (figs. 36, 37). As suggested by Strumpf (1898), the 
crumpling of the nucleus (fig. 37) in young sieve tubes may be mistaken for 
fragmentation. 
In conclusion, it is to be emphasized that this paper merely presents the 
results of a reconnaissance of cytological phenomena in the cambium. Its 
primary object is to outline salient features of the more striking phenomena 
encountered, and to pave the way for subsequent and more detailed in- 
vestigations, on the part both of the writer and of others who may become 
interested in the cambium as an unusually favorable medium for the study 
of certain cytological and physiological problems. 
I am indebted to my friends, Doctors C. E. Allen, R. A. Harper, W. J. 
V. Osterhout, and C. E. McClung for a number of helpful suggestions. 
Summary and Conclusions 
I. The initials of the lateral meristem or cambium, which may attain 
a length of more than 9,000 microns and a capacity exceeding 10,000,000 
cubic microns, are uninucleate. The working sphere of their nuclei must 
extend in certain cases for a distance of several thousand microns. 
