50 LECTURE IV. 



roots, water-roots, air-roots, and the roots of parasitic plants 

 which penetrate into their hosts. The difference of organi- 

 sation between land- and water-roots depends upon the fact 

 that the former have to absorb water in opposition to the 

 force with which it is retained by the soil, whereas the latter 

 absorb only free water. The experiments of Sachs clearly 

 indicate this difference. He found that if the roots of a land- 

 plant be kept immersed in water, they will persist for a time 

 and supply the plant with water ; sooner or later new roots 

 are developed which are adapted for the absorption of water 

 under these conditions which are, in fact, water-roots and 

 the original roots, at least the younger ones, die. When a 

 land-plant is grown from the first under these conditions, it 

 forms only water-roots. If such a plant be now transferred to 

 soil it will wither, doubtless because its roots are incapable of 

 taking up the hygroscopic water. 



Air-roots are found principally in monocotyledonous 

 plants, such as Orchids and other Epiphytes. That they 

 can absorb water is shewn by the fact that plants which 

 have no other means of supply, continue to grow. They 

 present evident peculiarities of structure. In Orchids the 

 air-root is invested by a membrane, the velamen, consisting 

 of several layers of cells containing air, the irregularly 

 thickened walls being perforated ; in other cases the cortical 

 parenchyma of the root is loose and spongy, and the epi- 

 dermal cells are produced into a number of long hairs. These 

 roots are thus enabled to retain any drops of water which 

 may fall upon them. This water will dissolve, or at least 

 assist in the solution of any mineral substances which may have 

 been deposited on the surface of the roots in the form of 

 dust, and thus it is that the plant obtains its supplies. 



From the researches of Unger and of Sachs it would appear that the 

 velamen of Orchids can condense watery vapour, and thus make it avail- 

 able for absorption. 



Air-roots cannot adapt themselves to an existence in soil 

 or in water ; according to Chatin, they die if they penetrate 

 into the one or the other ; and conversely, if the root of a 



