DEVELOPMENT OF SEED IN POLYGONACEAE 463 
and p. sagittatum, the edges of the cotyledons are pressed against an 
angle of the seed and the chalazal region, while in P. aviculare, P. 
tenue, Rumex crispus, and R. Acetosella, the back of the outer cotyledon 
presses against a face of the seed and the chalazal region. 
Rheum 
A single species, R. Rhaponticum, was taken into consideration. 
Its development to maturity corresponds in many respects to that 
of Polygonum Persicaria, but some differences are evident, which 
deserve special attention. 
The mature fruit is broadly three winged, a feature quite char- 
acteristic for this genus. The length of the fruit is 5 mm. and the 
width, exclusive of the wings, 3.6 mm. The inner wall of the ovary 
so indents the testa of the seed as to cause it to be very much longi- 
tudinally folded, as shown in figure 21. 
The aleurone layer (fig. 20, A) instead of being several cells thick, 
as in the type, consists of a single layer of cells. The mature embryo, 
occupying a central position (figs. 20 and 21) is relatively larger than 
the type, corresponding with the larger size of the seed. Stevens 
found the mature embryo of Fagopyrum esculentum also occupying a 
medium position in seed. Rheum and Fagopyrum are the only 
Polygonaceae according to the investigations up to the present time, 
possessing this character. The mature seed of R. Rhaponticum is 
abruptly narrowed at the apex, so that the short, massive, cylindrical 
hypocotyl practically fills the micropylar region of the seed, only the 
aleurone layer separating it from the testa (fig. 20). In R. palmatum, 
Lubbock (14, p. 436) found a similar condition, but in R. ofiicinale, 
he found the seed not abruptly narrowed at the apex. The broad, 
thin cotyledons (fig. 21, C), which are about two and one half times 
the length of the hypocotyl, do not extend to the base of the seed, as 
shown in figure 20. They are sometimes more or less folded longi- 
tudinally. Lubbock describes an exceptional condition in R. pal- 
matum in which the cotyledons are oblique to one another, and in 
one instance they were twisted at right angles to one another so as to 
occupy the three angles of the seed and thus attain the greatest 
possible size. 
A very interesting feature in R. Rhaponticum is the petioled condi- 
tion of the cotyledons at the base of which is a quite well developed 
plumule. In Polygonum, Polygonella, and Rumex, the cotyledons 
