334 VEGETABLE PHYSIOLOGY 



petioles respond by resuming their growth, so as always to 

 keep pace with the increased depth. Their structure- 

 resembles that of the stem in that they are composed of 

 turgid parenchyma and have little or no development of 

 woody tissue. They also contain conspicuous lacunae or 

 air channels (fig. 143). 



Vegetative reproduction is very common, branches 

 becoming detached from the plant, which speedily put out 

 adventitious roots of their own and form new plants. 



Their watery environment explains the peculiarity of 

 their structure. From the nature of their surroundings 

 and their power of absorbing liquid through their epider- 

 mis we can easily explain the absence of the woody tissue, 

 which we have seen to be, when present, especially devoted 

 to the conducting of water from the roots throughout the 

 plant. Their absorbing tissue being their whole super- 

 ficial investment, such conduction is not called for for 

 nutritive purposes. Their transpiration moreover is re- 

 duced to a minimum, and there is therefore no need of a 

 provision for the rapid current of water which is so essential 

 to the well-being of a terrestrial plant, in which this 

 function is so prominent. Their food materials reach them 

 dissolved in the water in which they live, and hence they 

 have no need of the complicated root system with its 

 absorbent root-hairs, which is so characteristic of a plant 

 growing in ordinary soil. Gaseous absorption takes place 

 through the general surface to a large extent, but this 

 direct supply is insufficient for respiration. The ordi- 

 nary arrangements for aeration, consisting of a network 

 of intercellular spaces freely in communication with nu- 

 merous stomata, are not exhibited by plants surrounded by 

 water. We have seen that many of them have no stomata, 

 the leaves being quite submerged ; others have relatively 

 few on the upper surfaces of the floating leaves. The 

 gaseous interchange between the interior and the exterior 

 is consequently greatly impeded. The large intercellular 

 lacunae form a mechanism by which this difficulty is 



