Parenchyma cells form the remainder of the spermoderm; and these are partially obliterated, so that the structure is not easily seen, appearing almost like a solid membrane. The raphe runs through the parenchyma found in the cleft of the berry.
The endosperm (Figs. 333; 338) consist of small cells in the outer part, and large cells, frequently as thick as 100 µ, in the inner part. The cell walls are thickened and knotted. Certain of the inner cells have mucilaginous walls which when treated with water disappear, leaving only the middle lamellae, which gives the section a peculiar appearance. The cells contain no starch, the reserve food supply being stored cellulose, protein, and aleurone grains. Various investigators report the presence of sugar, tannin, iron, salts, and caffein.
The embryo (Fig. 331, III) may be obtained by soaking the bean in water for several hours, cutting through the cleft and carefully breaking apart the endosperm. If it is now soaked in diluted alkali, the embryo protrudes through the lower end of the endosperm. It is then cleared in alkali, or in chloral hydrate. The cotyledons shown have three pairs of veins, which are slightly netted. The radicle is blunt and is about 3⁄4 mm in length, while the cotyledons are 1⁄2 mm long.
Fig. 336. Coffee. Sclerenchyma fibers of endocarp. x160. (Moeller)
The Coffee-Leaf Disease
The coffee tree has many pests and diseases; but the disease most feared by planters is that generally referred to as the coffee-leaf disease, and by this is meant the fungoid Hemileia vastatrix, which as told in chapter XV, destroyed Ceylon's once prosperous coffee industry. As it has since been found in nearly all coffee-producing countries, it has become a nightmare in the dreams of all coffee planters. The microscope shows how the spores of this dreaded fungus, carried by the winds upon a leaf of the coffee tree, proceed to germinate at the expense of the leaf; robbing it of its nourishment, and causing it to droop and to die. A mixture of powdered lime and sulphur has been found to be an effective germicide, if used in time and diligently applied.
Fig. 337. Coffee. Spermoderm in surface view. st. sclerenchyma; p, compressed parenchyma. x160. (Moeller) | |
Fig. 338. Coffee. Cross-section of outer layers of endosperm, showing knotty thickenings of cell walls. x160. (Moeller) | Fig. 339. Coffee. Tissues of embryo in section. x160. (Moeller) |
Value of Microscopic Analysis