ON THE STUDY OF ADAPTATION IN PLANTS. 183 



walls, mucilage, etc., raphides and other crystals of 

 calcium oxalate). 



The physical adaptations appear in the mechanical 

 construction of the tissues in the form of the plant organs 

 (leaves, roots, etc.), and in active response to stimulus. 

 As regards the adaptation of the tissues, the progress 

 made in this sphere has not been very extensive since 

 Haberlandt brought together, in his Physiologische Pflan- 

 zenanatomie, the results which had been arrived at up to 

 that date. It may, however, be worth while to refer to 

 the researches on the importance of intercellular spaces 

 in marsh and aquatic plants. They have been regarded 

 until recently as air-reservoirs, which provide the plants 

 with an internal atmosphere. But, at the same time, it 

 remained quite incomprehensible why the parts of marsh 

 plants which did not come in contact with the water or 

 the damp soil should contain large intercellular spaces. I 

 have endeavoured (in the work to which I have frequently 

 alluded) to point out that it is here principally a question 

 of providing the submerged parts with oxygen, of which 

 there is a proportionately smaller quantity in water than 

 in air. Oxygen is separated during the assimilation on 

 the part of the green portions, it reaches the intercellular 

 spaces, and can so easily be distributed, as the intercellu- 

 lar spaces represent either a connected system, or else 

 the diaphragms which separate them are easily pene- 

 trated by gases. If the tissue containing the numerous 

 intercellular spaces owes its origin to the action of a 

 cork-cambium, we have the Aerenchyma, which clothes 

 the submerged parts of certain plants in a thick, white 

 covering ; and the light, air-containing wood of some 

 plants fulfils exactly the same function, and it was for- 

 merly believed to serve as ''swimming-wood". These 

 adaptive modifications are entirely wanting among the 

 greater part of our native plants. 



I should exceed the space allotted to me if I were to 

 attempt to even mention the most important results of 

 the researches on the physical adaptation of the external 

 organs of plants. So many valuable contributions have, 



