The starch grains are of very variable size even in the same plant, and their form is as variable as their size, though there is a general resemblance in those of the same plant which allows of their being fairly easily identified after a moderate amount of practice. Sometimes the grains are found loosely packed in the interior of the cells, and are then easily recognised as starch grains by their peculiar form and the delicate lines with which they are marked; but in many places they are pressed so closely together that they assume an hexagonal shape under the microscope, and bear a close resemblance to ordinary twelve-sided cells. In other plants, again, the grains never advance beyond the very minute form in which they seem to commence their existence; and in some, such as the common oat, a great number of very little granules are compacted together so as to resemble one large grain.
There are several methods of detecting starch in those cases where its presence is doubtful; and the two modes that are usually employed are polarised light and the iodide of potassium. When polarised light is employed—a subject on which we shall have something to say presently—the starch grains assume the characteristic “black-cross,” and when a plate of selenite is placed immediately beneath the slide containing the starch grains, they glow with all the colours of the rainbow. The second plan is to treat them with a very weak solution of iodine and iodide of potassium, and in this case the iodine has the effect on the starch granules of staining them blue. They are so susceptible of this reaction that when the liquid is too strong the grains actually become black from the amount of iodine which they imbibe.
Nothing is easier than to procure starch granules in the highest perfection. Take a raw potato, and with a razor cut a very thin slice from its interior, the direction of the cut not being of the slightest importance. Put this delicate slice upon a slide, drop a little water upon it, cover it with a piece of thin glass, give it a good squeeze, and place it under a power of a hundred or a hundred and fifty diameters. Any part of the slice, provided that it be very thin, will then present the appearance shown in Plate III. Fig. [9], where an ordinary cell of potato is seen filled loosely with starch grains of different sizes. Around the edges of the slice a vast number of starch granules will be seen, which have been squeezed out of their cells by pressure, and are now floating freely in the water. As cold water has no perceptible effect upon starch, the grains are not altered in form by the moisture, and can be examined at leisure.
III.
| FIG. | |||
| 1. | Laurel leaf, transverse section | 27. | Do. Heath, another species |
| 2. | Starch, Wheat | 28. | Pollen, Furze |
| 3. | Do. from Pudding | 29. | Do. Tulip |
| 4. | Do. Potato | 30. | Petal, Pelargonium |
| 5. | Outer Skin, Capsicum pod | 31. | Do. Periwinkle |
| 6. | Starch, Parsnip | 32. | Do. Golden Balsam |
| 7. | Do. Arrow Root, West Indian | 33. | Do. Snapdragon |
| 8. | Do. “Tousles Mois” | 34. | Do. Primrose |
| 9. | Do. in cell of Potato | 35. | Do. Scarlet Geranium |
| 10. | Do. Indian Corn | 36. | Pollen, Crocus |
| 11. | Do. Sago | 37. | Do. Hollyhock |
| 12. | Do. Tapioca | 38. | Fruit, Galium, Goosegrass |
| 13. | Root, Yellow Water-Lily | 39. | A hook of ditto more magnified |
| 14. | Starch, Rice | 40. | Seed, Red Valerian |
| 15. | Do. Horsebean | 41. | Portion of Parachute of same, more magnified |
| 16. | Do. Oat | 42. | Seed, Foxglove |
| 17. | Pollen, Snowdrop | 43. | Do. Sunspurge |
| 18. | Do. Wallflower | 44. | Parachute, Dandelion seed |
| 19. | Do. Willow Herb, a pollen tube | 45. | Seed, Dandelion |
| 20. | Do. Violet | 46. | Do. Hair of Parachute |
| 21. | Do. Musk Plant | 47. | Do. Yellow Snapdragon |
| 22. | Do. Apple | 48. | Do. Mullein |
| 23. | Do. Dandelion | 49. | Do. Robin Hood |
| 24. | Do. Sowthistle | 50. | Do. Bur-reed |
| 25. | Do. Lily | 51. | Do. Willow Herb |
| 26. | Do. Heath | 52. | Do. Musk Mallow |
III.
On focusing with great care, the surface of each granule will be seen to be covered with very minute dark lines, arranged in a manner which can be readily comprehended from Fig. [4], which represents two granules of potato starch as they appear when removed from the cell in which they took their origin. All the lines evidently refer to the little dark spots at the end of the granule, called technically the “hilum,” and represent the limits of successive layers of material deposited one after another. The lines in question are very much better seen if the substage condenser be used with a small central stop, so as to obtain partial dark-field illumination. Otherwise they are often very difficult of detection.