Fig. 28.—Root of an Eastern (Wisconsin) Fruit Tree.
(Photograph by Prof F. H. King.)
Root-system in the Humid Region.—It is quite otherwise where a dense subsoil not only obstructs mechanically the deep penetration of any but the strongest roots, but at the same time is itself too inert to provide sufficiently abundant nourishment apart from the surface soil, which is there the portion containing, alongside of humus, the bulk of the available plant-food, and in which alone the processes of absorption and nutrition find the proper conditions; such as access of air and the ready and minute penetration of even the most delicate rootlets and root-hairs. The largest and most active portion of the root-system being thus accumulated in the surface soil, it follows that unless the latter is constantly kept in a fair condition of moistness, the plant must suffer material injury very quickly; hence the often fatal effects of even a few weeks’ drought. The same occurs in the arid region when often-repeated shallow plowing has resulted in the formation of a “plowsole” which prevents the deep penetration of roots; when a hot “norther” will often in a short time not only dry the plowed soil, but will heat it to such extent as to actually bake the roots it harbors. Under the same weather-conditions an adjoining field, properly plowed, may almost wholly escape injury.
Fig. 29.—Prune Tree on Peach Root, at Niles, Cal.
Comparison of root development in the arid and humid regions.—Figures 28, 29 given here show the differences as actually seen in the case of fruit trees as grown in Wisconsin and California, respectively, both in the absence of artificial water-supply.
Adaptation of humid species to arid conditions.—Figures, in No. 30, show the root systems respectively of the riverside grape (Vitis riparia) as grown in the Mississippi Valley states, and the natural development as found in the Rock grape of Missouri and also in the wild grape vine of California. It will be noted at once that the latter directs its cord-like roots almost vertically from the first, until it reaches a depth varying from 12 to 18 inches, where it begins to branch more freely, but still with a strong downward tendency in all. The roots of the riverside grape, on the contrary, tend to spread almost horizontally, branching freely at the depth of a few inches and manifestly deriving its supply both of plant-food and moisture mainly from the surface soil. It is curious to observe the behavior of this vine when cuttings are planted in California vineyards as a resistant grafting-stock. Its first roots are sent out horizontally, very much as is its habit in the East, so long as the soil moisture is maintained near the surface. But as the season advances, the more superficial rootlets are first thrown out of action by the advancing dryness and heat of the surface soil, and many finally die the first year.
Fig. 30.—Root Growth of Resistant Grape Vines.
Not unfrequently the entire root system developed by the uppermost bud perishes; but usually its main roots soon begin to recede from the threatening drought and heat of the surface, curving, or branching downward in the direction of the moisture supply, and without detriment to their nutrition because of the practical identity of the surface soil and subsoil. As the portions of the roots near the surface thicken and mature, their corky rind soon prevents their being injured by the arid conditions to which they are subjected; while the root-ends, finding congenial conditions of nutriment and aeration in the moist depths, develop without difficulty as they would in their humid home. Practically the same process of adaptation takes place in every one of the trees, shrubs, or perennials belonging to the humid climates, until their root system has assumed nearly the habit of the corresponding native vegetation.
The photograph of the roots of a hop plant, grown on bench lands of the Sacramento river, shows the roots extending to 8 feet depth, but where broken off the main root is still nearly two millimeters in thickness, proving that it penetrated at least two feet beyond the depth shown in [figure 31.]