SWEET-CLOVER SEED. \) 



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. 





Pure 



Cane sugar in solution (per cent). 



MelilotUbalba. water _ 



8 



12 



18 



24 



30 



35 



45 



55 



Germination of pollen percent. . 33 



23 



64 



40 



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— BuU. 844 2 



