------------+---------------------+-----------------+-------------- PLANT. | Area of surface | Area of | No. of times | without root-hairs. | root and hairs. | greater. ------------+---------------------+-----------------+-------------- Maize, | 4.52 sq. mm. | 25.13 sq. mm. | 5.5 Pea, | 4.71 sq. mm. | 58.33 sq. mm. | 12.4 Scindapsus, | 14.02 sq. mm. | 261.9 sq. mm. | 18.7 ------------+---------------------+-----------------+-------------- --which sufficiently establishes the general proposition that the area of the root-surface is enormously increased by these hairs. But this does not give us any definite idea of the length of the cylinders of soil explored by these surfaces, until we find that plants such as an ordinary sunflower, hemp, or vegetable-marrow may have roots penetrating into a cubic meter of soil, in all directions, and so closely that probably no volume so large as a cubic centimeter is left unexplored. Clark found by actual measurement that the roots of a large gourd, if put end to end, extended over 25 kilometers, and Nobbe gives 520 meters for the roots of a wheat. Vetches may go nine feet deep, and oats more than three feet. The Sal, a tree of the forests of India, has roots which penetrate to a depth of 50 to 60 feet. Some rough notion of the lengths, superficies and penetrating capacities of the roots of a large tree may be gathered from the above, but it is doubtful whether we can form any adequate ideas as to the millions of root-hairs which must be developed along the course of these subterranean boring organs. One of the most striking results of modern enquiry into these matters, is the discovery that the number and superficial area of these root-hairs, on one and the same plant, may vary to a large extent according to the structure, as it were, of the soil, and the degree of moisture it is capable of retaining; or, with the same soil, according to the amount of water which it receives and holds. Correlations have also been observed between the development in length and surface of the rootlets themselves. The following illustrations will suffice to show this: Six young wheat-plants in soil kept constantly wet, developed roots the total length of which measured 365 mm. each, on the average, and almost devoid of root-hairs. Six similar plants in soil only moderately moist, averaged 668 mm., and were well furnished (though not densely covered) with root-hairs. Six similar plants in soil which would be termed dry, averaged 371 mm., but were densely covered with rich crops of root-hairs. Further researches have shown that the conditions which rule the development of the root-system and root-hairs in the soil are very complex, and not always easy to trace. The most general statements we can make are the following: