THE PECTINATE ORGANS OF THE WATER-CHESTNUT. 87 



is no differentiation into palisade tissue, or spongy tissue, the vascular 

 kindle being simply imbedded in a parenchymatous tissue. The 

 epidermis consists of elongated cells (Fig. 4) which are possessed of 

 extremely thin outer walls. This is the reason why the submerged organs 

 begin to wither already after some minutes when exposed to the open 

 air. Stomata could not be detected and most of the chlorophyll bodies 

 were found to be located not in the tissue below the epidermis, but 

 chiefly in the epidermal cells themselves. 



Some of these characters seem to be incompatible with each other ; 

 but a closer examination of the relations between those internal and 

 external contrivances and the surrounding factors will show that those 

 organs are nothing but the most excellent adaptations to the medium in 

 which they live. A differentiation of the mesophyll into palisade tissue 

 and spongy tissue would be quite superfluous, as leaves submerged in 

 water do not transpire. That the chlorophyll corpuscles are chiefly 

 contained in the epidermal cells, is due to the circumstance that the 

 light to which the leaves are exposed is not very intense, because it has 

 to pass through the water before it reaches the tissues. The gradual 

 decrease of the intensity of the light with the depth of the water is 

 shown to evidence in the colour of the submerged organs. The chroma - 

 tophores of the uppermost organs are dark- green : they become paler 

 and paler as we follow the stem in a downward direction, and assume 

 finally a brownish colour, where every assimilatory function of the 

 chromatophores is rendered impossible for want of light. On the one 

 hand the absence of stomata seems to indicate that respiration does 

 not take place ; but, on the other, the presence of chlorophyll corpus- 

 cles is a sufficient proof that the processes of photo-synthetic assimi- 

 lation of carbon dioxide and, consequently, of respiration are going on 

 in the usual way. It is just in order to facilitate these processes and to 

 give at the same time a compensation for the absence of special respira- 

 tory organs, that the outer walls of the epidermal cells are so extremely 

 thin as to allow not only the exchange of gases, but also the passage of 

 salts dissolved in water. In order that the organs may present as large 

 a surface as possible, they do not develop in the form of flat expansions, 

 as the floating leaves do, but are much divided and extended into 

 capillary processes. 



The question now arises, whether we shall call these organs roots, 

 leaves, stipules, or branches. If we take into consideration all the 



