BREATHING-ROOTS AERIAL ROOTS 481 



Avicennia officinalis and Lcujuncularia racemosa, the periderm, which is 

 several layers in thickness, is interrupted by structures agreeing in all 

 essentials with ordinary lenticels. The breathing-roots of Sonneratia 

 acida are surrounded, according to Goebel, by three-layered sheets 

 of cork, which alternate with one- to two-layered zones of rounded 

 cells; the latter resemble the complementary cells of lenticels, while 

 the thin sheets of cork may be compared to the closing layers of those 

 organs. As Karsten has remarked, the root of Sonneratia may be 

 said to be invested by a continuous pneumathode ; it is as if a single 

 large lenticel had spread over the whole surface of the organ. Accord- 

 ing to Jost, the breathing-roots which occur in certain Palms, notably 

 in the genus Phoenix, are provided with pneumathodes of a peculiar 

 kind. When viewed with the naked eye, they appear as white powdery 

 areas, which may be confined to the dilated root-tip, or may, on the 

 other hand, be located on annular swellings further back. Micro- 

 scopic examination shows, that at these points the subepidermal fibrous 

 tissue is replaced by a mass of rounded sclerenchymatous elements, 

 separated by narrow intercellular clefts, which in turn is overlain by a 

 layer of thin-walled " spongy tissue," likewise richly provided with 

 intercellular spaces. If the experiment described above in connection 

 with lenticels is performed upon these roots, it is found that here 

 also a very slight excess of pressure suffices to force air through the 

 pneumathodes. 



2. The pneumathodes of aerial roots. 23 '' 



The pneumathodes which occur on the aerial roots of Orchids were 

 described in detail by Leitgeb ; but Schimper was the first to give 

 a correct account of their function. To the naked eye they appear as 

 fairly well-defined white spots, which are most conspicuous when the 

 velamen is saturated with water. As a rule they are elongated 

 parallel to the long axis of the root, and arranged in fairly regular 

 longitudinal series ; in cylindrical roots they are evenly distributed on 

 all sides, whereas in flattened dorsiventral roots, such as those which 

 constitute the photosynthetic organs of Aeranthus fasciola and Taenio- 

 phyllum Zollingeri, they are confined to the side which is next the 

 substratum, and shaded. 



Each of these pneumathodes comprises three distinct parts, 

 which pertain respectively to the velamen, the exodermis, and the 

 cortical parenchyma. The outermost portion consists of a wedge- 

 shaped mass of velamen cells, distinguished by the fact that the spiral 

 thickening fibres are unusually numerous, or that the cell-walls are 

 uniformly thickened except for elongated pits. The outstanding 

 physiological feature of these cells is the tenacity with which they 



2h 



