138 THE STRUCTURE OF FLOWERS. 



to the particles of soil.* Cbatin noticed the production of 

 hairs when the roots came in contact with any obstacle ; f 

 but Dr. M. T. Masters observes that the obstacle alone in 

 their case is insufficient without moisture, for he found that 

 the roots of Mustard-seed could penetrate a stiff clay, but did 

 not develop any root-hairs until they came in contact with 

 the sides of the pot — "Wherever there was a thin film of 

 water investing a stone or the sides of a porous flower-pot 

 or a plate of glass, there the root-hairs abounded." 



Besides a nutrient or moist medium, actual growth in 

 water may enormously increase the length and quantity of 

 root-hairs ; as may be seen in the dependent roots of floating 

 plants of Hydrocharis, etc. ; or in the hypertrophied con- 

 ditions of the roots of grasses when growing in water. 



That epidermal trichomes may be due to the irritation of 

 insects is clearly seen by their appearance within the cavities 

 of certain galls. J In the case, for example, of a very com- 

 mon one on willows, the leaf bulges out below and forms 

 a sort of bag, open or closed above. The tissues become 

 hypertrophied though the epidermis and palisade cells are 

 still recognizable lining the cavity. The leaf has scattered 

 hairs on both sides ; but within the cavity much larger hairs, 

 rich with protoplasmic or other matters, project from all 

 sides into the interior. Some are straight, others curved, 

 club-shaped, or with irregularly swollen ends, not unlike the 

 forms produced on climbing roots by contact with a foreign 

 body. Again, the crimson "spangles," so common on the 

 underside of Oak-leaves, are covered with stellate clusters 



* Sachs' Phys. of PI. (Eng. ed.), 1887, fig. 12, p. 19. 



t Mem. Soc. Nat. Sci., Cherbourg, 1856, p. 5 ; referred to by Dr. 

 M. T. Masters in Notes on Root-hairs, etc., Jonra. Roy. flort. Soc, vol. v., 

 p. 174. 



X Caused by species of Nematus. 



