The differences in the quantity of ash contained in different parts of plants are obviously intended to serve a useful purpose, and it is interesting to observe that the wood which is destined to remain for a long period, sometimes for several centuries, a part of the plant, contains the smallest proportion, and it is not improbable that what it does contain is really due, not to the actual woody matter itself, but to the sap which permeates its vessels. By this arrangement but a small proportion of these important mineral matters, which the soil supplies in very limited quantity, is locked up within the plant, and those which are absorbed, after circulating through it, and fulfilling their allotted functions, are accumulated in the leaves, and annually returned to the soil. The different proportions of mineral matters contained in the individual organs of plants is most strikingly illustrated when parallel experiments are made on the same species; but the number of instances in which a sufficiently extensive series of analyses has been made to show this, is comparatively limited, and is confined to the oat, the orange-tree, and the horse chesnut--each of which has formed the subject of a very elaborate investigation. The following table gives the results obtained on the oat:-- +-------------+-----------+---------+-----------+-------+-------+------+ | | Hopetoun | Hopetoun| Potato | Black | Sandy | | | | Oats, | Oats, | Oats, | Oats, | Oats, | Mean.| | | North- | Fife- | North- | Edin- | Fife- | | | | umberland.| shire. | umberland.| burgh.| shire.| | +-------------+-----------+---------+-----------+-------+-------+------+ | Grain | 2.14 | 1.81 | 2.22 | 2.11 | 1.76 | 2.00| | Husk | 6.47 | 6.03 | 6.99 | 8.24 | 6.03 | 6.75| | Chaff | 16.53 | 17.23 | 15.59 | 19.19 | 18.97 | 16.06| | Leaves | 8.44 | 7.19 | 14.59 | 10.29 | 15.92 | 10.88| | Upper part | | | | | | | | of straw | 4.95 | 5.44 | 9.22 | 8.25 | 11.0 | 7.77| | Middle part | | | | | | | | of straw | 6.11 | 5.23 | 7.41 | 6.53 | 9.01 | 6.66| | Lower part | | | | | | | | of straw | 5.33 | 5.18 | 9.76 | 7.11 | 7.30 | 6.93| +-------------+-----------+---------+-----------+-------+-------+------+