106 FIELD AND FOREST. 



digging the ground at various seasons it was only very rarely that plant 

 remains were found in the subsoil, and probably they got there by ac- 

 cident. 



With reference to the structure of the worm tubes, some interesting 

 facts were established in these researches. In humus their character 

 is difficult to make ont, owing to the looseness of the mass. In sand 

 they proceed almost vertically downwards three, four, or even six feet, 

 whereupon they often extend some distance horizontally ; more fre- 

 quently they terminate without bending. At the end of the tube the 

 worm is found with his head upwards, while round about him the tube 

 is lined with small stones. On the sandy wall of the tube one observes 

 more or less numerous black protuberances which make the sand fer- 

 tile. These are the secretions of the worm, which after being re- 

 moved out of a tenanted tube, are found next morning replaced by 

 fresh matter. They are observed after a few days, when a worm is 

 put in a vessel with clean sand, and allowed to make a tube for itself. 

 Older abandoned tubes are pretty regularly lined with the earth 

 formed by the worm, and some passages are densely filled with black 

 earth. This black substance appears to diffuse somewhat ino the 

 sand. 



In about half of the tubes not quite newly made, M. Hensen found 

 roots of the plants growing at the surface, in the most vigorous devel- 

 opment, running to the end of the tube and giving off fine root hairs 

 to the walls, especially beautiful in the case of the leafy vegetables and 

 corn. Indeed such tubes must be very favorable to the growth of the 

 roots. Once a root fibre has reached such a tube it can, following 

 the direction of gravity, grow on in the moist air of the passage, 

 without meeting with the least resistance, and in it finds moist, loose, 

 fertile earth in abundance. 



The question whether all the roots found in the under soil have 

 originally grown in the tubes of worms, cannot be answered with cer- 

 tainty. It is certain that the roots of some plants penetrate them- 

 selves in the sand, but not to great depths. M. Hensen is of opinion 

 that the tap roots, and in general such root forms as grow with a 

 thick point, can force a path for themselves, while the fine and flex- 

 ible suction roots have difficulty in obtaining a path into the depths 

 other than that previously made for them. Roots of one year's growth 



