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I. W. BAILEY 
initials and fiber tracheids — which is closely correlated with the development 
and specialization of vessels — results in a reduction in volume of these 
elements, but the decrease in length of the vessel-segments frequently is 
more than compensated for by an increase in their cross-sectional area. 
Thus, there is less contrast between the volume of the tracheids in gymno- 
sperms and that of the vessel-segments in dicotyledons than there is between 
the size of the cambial initials* in the two groups of plants. 
Significance of Size Variations in Cambium and Xylem 
These fundamental types of cell size variations, and concomitant fluc- 
tuations in form and structure, are significant in the investigation of a 
number of cytological, morphological, and physiological problems, as 
well as in the study of the identification and mechanical properties of timber, 
and will be discussed in greater detail in subsequent papers. 
In view of the numerous factors or complexes of factors which affect the 
dimensions and volume of cells, it is not surprising that contradictory 
conclusions have been reached by different investigators who have attempted 
to generalize concerning cell size after limited induction. The data at 
hand indicate very clearly that the undifferentiated, actively dividing and 
growing cells of the lateral meristem or cambium may vary greatly in size 
in certain plants and remain relatively constant in others. Therefore, 
very different conclusions concerning the constancy of cell size or of cell 
number may be expected from intensive experimental investigations, depend- 
ing upon the particular plant or portion of a plant which is selected for 
study. Similar discrepancies may be expected concerning body size and 
cell size. Depauperate plants (physiological dwarfs) frequently have 
smaller tracheary cells and cambial initials than individuals of normal 
stature, indicating a close correlation between cell size and body size. 
On the other hand, a large dicotyledon may be composed of much smaller 
cells than a small conifer or dicotyledon of similar age, suggesting that 
variations in cell size are independent of fluctuations in body size. 
Sachs (1892, 1893, 1895) and Strasburger (1893) almost simultaneously 
called attention to the fact that undifferentiated, actively dividing and grow- 
ing cells of plants, such as occur in embryonic and meristematic tissues, 
are relatively minute, and concluded that this is undoubtedly due to the 
fact that the working sphere of the nucleus is very restricted. Strasburger 
found that in terminal meristems the ratio between the average diameter 
of the nuclei and of the cells is as 0.003-0.16 mm: 0.005-0.24 mm., or 2 : 3, 
and Sachs pointed out that, although plants vary enormously in their linear 
dimensions (0.001 mm. to zoom.) , the size of their constituent cells is relatively 
constant (0.001 to 0.05 mm.). Winkler (1916) reaches similar conclusions. 
He states that in meristematic somatic tissues the cells are of nearly uniform 
size and contain the diploid number of chromosomes, whereas in non- 
meristematic somatic tissues, in which multinucleate protoplasts, nuclear 
