THE VEGETATIVE ORGANS 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 injured, and set into wa- 
ter, it does not suffer any hindrance in growth, as Sachs 
has found by late experiments, (Hxperimental Physi- 
ologie, p. 177.) Ordinarily, the suspension of growth and 
decay of fibrils and rootlets is due, doubtless, to the 
mechanical injury they suffer in removing from the soil. 
Again, when a plant 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 Zaurus 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. (Der Baum, p. 172.) 
The formation of air-roots may be very easily observed by filling a tall 
vial with water to the depth of half an inch, inserting therein a branch of 
a common house-plant, the Zradescantia zebrina, so that the cut end of 
the stem shall stand in the water, and finally corking the vial air-tight. 
The plant, which is very tenacious of life, and usually grows well in 
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. Ina few days, if the temperature be 70° or there- 
about, air-roots will be seen to issue from the joints of the stem. These 
