HISTOLOGY OF PHLOEM IN WOODY ANGIOSPERMS 
Robinia (fig. 35), and Sambucus (figs. 41, 42). It is further shown in 
the two figures of Robinia (34, 32), which are taken from tangential 
sections of the phloem near the cambium in a four-year seedling and a 
mature tree, respectively. Figure 34 shows that the cambium is de- 
veloping almost entirely into parenchyma cells, whereas figure 32, 
representing the condition of a homologous portion of the phloem of 
the mature plant, shows the formation of an abundance of sieve tubes 
in addition to the parenchyma. 
No seedlings were found in which the number of sieve tubes as 
compared with the number of parenchyma cells was not much smaller 
than in the mature plant. In fact, in some of the one-year seedlings 
this condition was so extreme that no sieve tubes could be found even 
after long search with the oil immersion lens (e. g., Tilia and Robinia). 
This, of course, does not mean that sieve tubes are entirely absent, but 
it does at least indicate that they are very rare. 
In many of the younger seedlings examined, as indicated through- 
out the description, the few sieve tubes that were found were poorly 
developed. The sieve tubes themselves were very small (see figs. 
4, 5, 12, 35) ; the lattice was very faint or not discernible, and the sieve 
plates were frequently either without distinct pores or with very 
minute pores, and were not placed with any regularity upon the walls. 
In some cases the sieve tube elements were so much like parenchyma 
in size and shape that they could not easily be distinguished. For 
example, in the two-year seedlings of Robinia ready recognition of the 
sieve tubes would be impossible, were not red-staining slime strands 
present in these elements. 
The phloem of the one-year-old twigs of mature plants shows many 
of the features found in seedlings, although they are not so extreme. 
Thus the first annual ring of phloem generally shows poorly developed 
sieve tubes without lattice and sometimes without well-developed 
sieve plates. The sieve tubes are fewer in number, as compared with 
the number of parenchyma cells, than in the adult, and a greater pro- 
portion of the sieve plates are transverse. The size of the sieve plates, 
also, is much smaller than in the mature plant, as shown in Juglans 
nigra. Here there is an increase in the size of sieve plates from 12-20 
in the two-year twig to 25-40 /.t in a twelve-year-old branch. 
The structure of the phloem of seedlings as indicated in the fore- 
going observations, especially as regards the small number and pooi 
development of the sieve tubes and the abundance of well-developed 
