SWEET-CLOVER SEED. 31 



ruthenium red for pectic substances; and sulphuric acid, Congo red, 

 and aniline blue for callose. 



Where very thin sections were necessary for detailed study of the 

 structure of the seed coat, pods in various stages of development 

 were collected, and after the usual preliminary treatment they were 

 embedded in paraffin and sectioned with the microtome. Micro- 

 chemical tests were made with these sections by using various specific 

 stains. Safranin was used to test for cutin, suberin, and lignin; 

 haematoxylin and methyl blue for cellulose ; methylene blue, methyl 

 violet B, mauvein, and ruthenium red for pectic substances; and 

 aniline blue and Congo red for callose. In studying some points 

 with reference to the pore system of the seed coat, it was necessary 

 to use free-hand sections of fresh pods. 



In studying the seed coat in relation to the absorption of water, 

 both perme'able and impermeable seeds were soaked in water solu- 

 tions of safranin, gentian violet, eosin, and haematoxylin, then dried 

 and embedded in glycerin gum for sectioning. Seeds were soaked 

 in stains dissolved in 95 per cent alcohol to test the penetration of 

 alcohol. It was evident that the seed coats did not act as a filter, 

 as the stains passed through them with the water or alcohol. 



STRUCTURE OF THE SEED COAT. 



There is very little endosperm present in mature seeds of Melilotus 

 alba or M. officinalis. That which is present is quite permeable to 

 water and therefore bears no relation to the impermeable seeds of 

 these plants. 



The outer layer of the seed coat, which is the modified epidermal 

 layer of the ovule, is known as the Malpighian layer. (PI. V, figs. 1 

 and 2.) The cells constituting this layer, commonly called palisade 

 cells, are the most highly modified cells of the seed coat. They are 

 very much elongated, their length varying in the different regions of 

 the coat, and their outer tangential walls and the outer portions of 

 their radial walls are so much thickened that their lumiua are con- 

 fined to the inner portion of the cells, sometimes occupying less than 

 half the length of the cells. The inner tangential walls and inner 

 portions of the radial walls are thickened just previous to the death 

 of the cells, the thickening sometimes being only slight and sometimes 

 so much as to leave only very narrow lumina. 



There is a very thin layer on the outer surface of the Malpighian 

 cells which has been called cuticle by previous investigators, but the 

 chemical composition of this layer and its perviousness to water 

 indicate that there is very little cutin present. This layer is probably 

 the primary epidermal cell wall rather than a deposit on the outer 

 surface of the wall. To determine this a study of the development 

 of the Malpighian cells is necessary. 



