HISTOLOGY OF PHLOEM IN WOODY ANGIOSPERMS 
northern flora, it seems not unreasonable to suppose that the facts 
revealed in this research give at least a fairly accurate indication of the 
conditions present in woody plants as a whole. 
In view of the facts brought out in this .-study and confirmed in 
literature, it cannot be said that there is definite evidence of gradual 
advance in the evolution of the sieve tube from type i to type 3; at 
least, if there is such an advance it does not in any way parallel our 
present ideas of phylogeny. Indeed, Hemenway does not appear to 
be justified in making his types for sieve-tube classification as indicated 
by his figures (shown earlier in this paper), because in that classification 
the oblique end wall with many sieve plates is correlated with well- 
developed lattice, or sieve plates upon the side walls and the transverse, 
end wall bearing a single plate with poorly developed lattice. From 
the description of material it is evident that no such correlation exists. 
For example, both Ulmus americana (fig. 17) and Eleagnus angusti- 
folia (fig. 20) have single transverse sieve plates, and at the same time 
well-developed lattice. A similar condition was also pointed out in 
Fagus sylvatica by Dippel (3, p. 255), and later for Ficus elastica by 
De Bary (2, p. 177). Cases where the end walls are oblique and the 
lattice not well developed are even more numerous, e. g., Nyssa syl- 
vatica, Alnus incana, Catalpa bignonioides, Fraxinus americana, and 
others. Here the end walls are very oblique with from four to fifteen 
plates, whereas the lateral sieve fields are not clearly differentiated, 
the side walls being almost without cellulose thickenings and closely 
set with pores. 
Table i shows in a graphic way that the sieve tube does not undergo 
a transition from type i to type 3 throughout the series of families as 
they are now placed in the evolutionary scale. It represents a partial 
list of the genera examined, arranged in phylogenetic order. Sieve 
tubes are classified according to the number of sieve plates on the end 
walls without reference to the side walls. As previously stated, num- 
ber I indicates the lowest type with many sieve plates upon the end 
wall and number 3 the highest type with a single transverse plate. 
It is, of course, understood that such a definite classification cannot be 
actually made since all manner of intergrading forms exist. Under 
vessel types the scalariform, or lowest, is referred to by number i, the 
porous, or highest, by number 3, and those forms which have both 
scalariform and porous vessels by number 2. Under parenchyma 
distribution the lowest, or terminal, position is indicated by number I, 
