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 permeable 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. (PL 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 lumina 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. 
