SWEET-CLOVER SEED. 
9 
of the seed coat extend around the ovule from these points, involving 
at first only the outer or epidermal layer of cells which form the 
malpighian layer. Later, the cells just beneath the malpighian layer 
form the osteosclerid layer. Accompanying or closely following the 
formation of the osteosclerid cells, the remaining cell layers of the 
outer integument become modified into the nutritive and aleurone 
layer, and the seed coat is fully formed. Meantime the inner integu- 
ment is practically all used as food. 
MATURE POLLEN OF SWEET CLOVER. 
The pollen grains of Melilotus alba and of M. officinalis are quite 
similar. Each grain contains three germ pores, and when viewed 
so that the pores are visible they present a slightly angled appearance. 
The average dimensions of the pollen of Melilotus alba and of M. offici- 
nalis are 26 by 32 microns and 24 by 30 microns, respectively, when 
measured in the positions shown in b in Plate I, figure 6. 
The walls of the pollen grains have cutin deposited in them, as 
shown by their reactions with Sudan III, alkanin, safranin, and 
chloriodid of zinc. The contents of the pollen grains give a distinct 
reaction when tested for fat, and Millon's reagent shows that also 
some protein is present. Tests for sugars and starch showed that 
these substances are not present in perceptible quantities in mature 
pollen grains, although some starch is present in immature pollen. 
GERMINATION OF THE POLLEN. 
The germination of the pollen of Melilotus alba permits considerable 
variation in moisture, as is illustrated in Table I. 
Table I. — Germination of the pollen of Melilotus alba in water and in solutions of 
cane sugar of different strengths. 
Melilotus alba. 
Pure 
water. 
Cane sugar in solution (per cent). 
8 
12 
18 
24 
30 
35 
45 
55 
Germination of pollen per cent . . 
33 
23 
64 
46 
60 
46 
31 
22 

The results given in Table I represent the average of 12 tests. 
Some of the pollen grains burst in pure water and in the weak cane 
sugar solutions, the percentage of bursting being greatest in pure 
water and decreasing as the percentage of sugar in the solution was 
increased. There was considerable variation in the percentages of 
germination in both water and in the solutions of different strengths, 
and at times there was very little bursting which was not accompanied 
by a high percentage of germination. The pollen tubes grew as 
rapidly in water as in any of the sugar solutions, some reaching a 
153321°— 20— Bull. 844 2 
