THE TEGETATIVB ORGASTS OF PLANTS. 257 



complete explanation, without recourse to the supposition 

 that soil and water-roots are essentially diverse in nature. 

 When a plant which is rooted in the soil is taken up so 

 that the fibrils are not broken or inj ured, and set into wa- 

 ter, it does not sufier any hindrance in growth, as Sachs 

 has found by late experiments. (Experimental Physi- 

 ologie, p. 177.) Ordinarily, the suspension of growth and 

 decay of fibrils and rootlets is due, doubtless, to the 

 mechanical injury they sufier in removing from the soil. 

 Again, when a jjlant that has been reared in water is 

 planted in earth, similar injury occurs in packing the soil 

 about the roots, and moreover the fibrils cannot be brought 

 into that close contact with the soil which is necessary for 

 them to supply the foliage with water ; hence the plant 

 wilts, and may easily perish unless profusely watered or 

 shielded from evaporation. 



The issue of water or soil-roots, either or both, from 

 the same plant, according to the circumstances in which it 

 is placed, finds something analogous in reference to air- 

 roots. As before stated, these chiefly occur on tropical 

 plants, or in shaded, warm, and very moist situations. 

 Schacht informs us that in the dark and humid forest ra- 

 vines of Madeira and Teneriffe, the Launis Canariensis, a 

 large tree, sends out from its stem during the autumn rains, 

 a profusion of fleshy air-roots, which cover the trunk with 

 their interlacing branches and grow to an inch in thick- 

 ness. The following summer, they dry away and fall to 

 the ground, to be replaced by new ones in the ensuing au- 

 tumn. {JDer Jiaum, p. 173.) 



The formation of air-roots may be very easily observed by filling a tall 

 Ti;il with water to the depth of half an inch, inserting therein a branch of 

 a common house-plant, the Trademantla zebrina, bo that the cut end of 

 the stem shall stand iu the water, and finally corking the vial air-tight. 

 The plant, which is very tenacious of life, and usually grows well iu 

 spite of all neglect, is not checked in its vegetative development by the 

 treatment just described, but immediately begins to adapt itself to its 

 new circumstances. In a few days, if the temperature be 70° or there- 

 about, air-roots will be seen to issue from the joints of the stem. These 



Digitized by Microsoft® 



