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362 L. H. MACDANIELS 
cate that the single transverse plate is the primitive condition. It 
seems more probable, however, that the small number of sieve plates 
in the sieve tubes of seedlings is due to the greatly reduced size of these 
elements in the young plants. If such is the case, the significance of 
the small number of sieve plates in the sieve tubes of seedlings of 
plants with the first and second types of sieve tubes in the adult, 
would be physiological rather than phylogenetic. 
The extent of the differences in size between the sieve tubes of the 
seedling and those of the adult is readily shown by the following series 
of measurements of the radial width of sieve plates upon the end walls 
of sieve tube elements and by the series of photomicrographs. 
Populus: one-year stem, fig. 4, 6-10 /x; three-year stem, fig. 5, 
12-20 fx; four-year stem, fig. 6, 15-25 /jl; mature, fig. 7, 30-45 /x. 
Juglans: one-year stem, fig. 12, 8-12 yu; four-year stem, fig. 14, 
6-18 m; mature, fig. 15, 25-40 fi. 
Robinia: two-year root, fig. 35, 7-10^1; four-year root, 10-15 /x; 
mature, fig. 36, 35-40 
Sambucus: three-year root, fig. 42, 8-18 fx; mature, fig. 43, 20-30 fx. 
Since a single sieve plate extends completely across the sieve tube 
radially, its measurement may be taken as an accurate indication of 
the width of the tube. The difference in length between the sieve 
tubes in seedlings and those in mature plants is usually apparent also, 
but is not so marked. In fact, such difference in length is present in 
the cambium cells themselves, as is shown by figures 31 and 33 which 
are of like magnification. The first of these is taken from the tan- 
gential section of the cambium of a four-year seedling, whereas the 
second comes from a similar position in a mature stem. 
These figures show that in passing from the one-year seedling to 
the adult, there is a gradual increase in the size of the sieve tube. 
Such small size as that indicated for the younger seedlings would make 
the presence of the adult number of sieve plates on the end wall im- 
possible, without extreme reduction either of the size or of the number 
of pores upon the sieve plate. As a matter of fact, such reduction in 
size and number of pores does occur but not to the extent necessary 
for the accommodation of so large a number of plates on an end wall. 
The small size of the sieve tubes in seedlings is correlated with the 
small number of these elements that are present in phloem of young 
plants, as compared with the number of parenchyma cells. This is 
graphically shown in Populus (figs. 4, 5, 6), Juglans (figs. 12-14), 
