374 
L. H. MACDANIELS 
side wall the pores are so minute that they could not be recorded by 
the camera, being mere crowded points of light even under high mag- 
nification. The sieve plates and lattice of the Salicaceae studied are 
of nearly identical type with those found in the Juglandaceae. The 
difference in the size of the pores in the sieve plates and in the lattice 
is further shown by the measurements given in table 3. 
If similarity to the gymnosperm type of sieve tube is found any- 
where in the angiosperms, it apparently is not in the Juglandaceae 
but rather in such genera as Pyrus and Sambucus, which do show sieve 
plates of like structure upon both the side and end walls (figs. 26, 27, 
44)- 
In conclusion, it may be said that the present research is not 
thought to refute the idea that sieve-tube type and type of vessel have 
phylogenetic significance. Either to confirm or to disprove those 
theories would require the examination of a vastly greater number of 
genera representing the floras of the world. Rather is it desired for- 
cibly to bring out the fact that the evolutionary significance of any 
character is often obscure and that sweeping generalizations cannot 
be made upon the examination of a limited amount of material, even 
though the observations may be accurate. That histological and 
anatomical characters will have a very important influence upon the 
future ideas of phylogeny cannot be doubted. In the case of most 
characters, however, it remains for anatomical work on a scale much 
greater than that hitherto attempted to show what the real significance 
of those characters may be. 
The more important results of this study may be briefly stated as 
follows : 
1. In the woody dicotyledons, there is no fundamental difference 
in type between the sieve tubes in the phloem of seedlings and those 
in the mature plant, existing differences being mainly those of size. 
2. In the phloem of seedlings the number of sieve tubes as com- 
pared with the number of parenchyma cells is much smaller than in 
mature phloem. 
Table 3 
Juglans nigra . . . . 
Populus deltoides 
Salix nigra 
1.8-3.5 microns 
3-5-5-5 
2-3 
.5-.6 micron 
.5-.6 " 
A--5 " 
Summary 
