8o THE PHYSIOLOGY OF PLANTS 



rounding off of the cortical cells, which cause the root-tip to 

 become dilated into a knob. Below this spongy tissue the 

 usual sclerenchymatous ring, which forms a protective sheath 

 to the roots, is missing. If such an upward-growing root is 

 cut off and is hermetically sealed into the shorter arm of a 

 U-shaped glass tube, by pouring mercury or by blowing into 

 the other arm, air can be forced through the root-tip. This 

 proves that the aeration of the whole root takes place through 

 the mealy-looking tip. This means of aeration by specially 

 modified roots, which are termed " pneumathodes," occurs in 

 many species of Phoenix, Livistona, Cocas, Chamsedorea, Pan- 

 danus, &c. In the last-mentioned palms the respiratory 

 organs are found on thick horizontal roots in the form of 

 small wart-like lateral roots, scarcely I inch in length ; in 

 Pandanus the " pneumathodes " are very small warts on the 

 large prop-like roots. 



The explanation of this phenomenon, to which little atten- 

 tion has been paid, of roots being sent up out of pots, is to be 

 found in the sensitiveness of these organs to gases,^ which has 

 now been established. The gas which produces the abnormal 

 growth in this case is the oxygen of the air, which is driven 

 out of the interstices of the soil by watering. The more it is 

 watered, the closer the soil, and the damper the pot is kept, 

 the more the roots will grow towards the upper regions of the 

 pot, where there is more oxygen, and even grow right out of 

 the soil. If the soil is only kept moderately moist, so that 

 the roots can obtain the necessary supply of air within the 

 soil, plants which usually direct their roots upwards will pro- 

 duce their pneumathodes underground. It is their desire for 

 oxygen which causes the roots to grow towards the inner 

 surface of the pots, and this points out the necessity of 

 occasionally washing" the outside of the pots to keep their 

 pores open. 



^ Aerotropism. 



