321 



always so uniformly constructed must also always perform 

 the same principal function ; he admits, however, that the buoy- 

 ancy of the floating organs may be increased by this tissue, but 

 does not depend upon it. 



1 am not prepared to enter into a discussion of the general 

 function of the " aerenchyma " — which term is, at all events, a 

 very significant one — but must add the result of some observa- 

 tions, referring to NescBa only, made with particular regard to the 

 general statements in the above mentioned paper. It seems to 

 me that, above all, it is very important to answer the question : 

 Under what conditions, when and where does this tissue make its 

 first appearance ? In Nesc^a, as we have seen, it is first detected 

 at the point of contact of the apex of the stem with the surface 

 of the water. There it increases quite rapidly, while the portion 

 of the stem resting on the water often produces leaves and 

 branches. The latter are frequently surrounded with floating 

 tissue up to one or two inches above the surface of the water 

 (Fig. i). We are not justified in assuming that at this early stage, 

 when the whole stem is still exposed to the atmosphere, the 

 necessity of obtaining more oxygen causes the plant to produce 

 the aerenchyma, while it seems quite natural to suppose that it 

 is advantageous to the plant to be enabled to keep the ends of its 

 stems afloat, and let them continue their vegetative and repro- 

 ductive work until they have rooted and established themselves 

 as independent plants. 



The view that the aerenchyma of 7W^-^^ is chiefly a ** float- 

 ing " tissue, is perhaps supported, to some extent, by the obser- 

 vation that the meristem producing it is first noticed at the sides 

 and at the upper part of the horizontal floating stem, and that, 

 consequently at a later stage, the aerenchyma is more copiously 

 developed at those places than on the lower side, where the roots 

 grow. The first longitudinal crevices invariably appear at some 

 distance from the adventitious rootlets, parallel with the line 

 along which they grow, and higher up on both sides, enclosing a 

 broad zone of primary bark along the upper side of the stem. 

 It is easily seen that this position of the aerenchyma will give the 

 stem the greatest possible buoyancy. 



When the horizontal stem has become submerged and has 



