ADAPTATIONS FOR AERATING THE ROOTS OF TREES. 433 



lion is in all probability the protection of the chlorophyll tissue proper 

 against theeffectof too much heat and light by removing it thus much 



farther from the surface of the leaf. 



7. Palisade. — The value of palisade tissue in reducing transpira- 

 tion was discussed under "Adaptations to Environment in the Strand 

 Vegetation." 1 It is rather strongly developed in the following plants 

 of the Dismal Swamp: Smilax laurifolia (2 layers), Magnolia virgin- 

 iana (2 layers), Persea pubescens (2 layers), Ldquidambar styraciftua 

 (2 layers), Ilex glabra (4 layers), Ilex lucida (3 layers), Acer rubrum 

 var. (only one layer, but the cells are so high as to form more than 

 one-half the thickness of the mesophyll), Leucothoe axillaris (2 layers), 

 Leucothoe racemoxa (2 layers), Kalmia angustifolia (2 layers), Gel- 

 semium sempervirens (3 layers, but only the uppermost very com- 

 pact). 



AKUATINO ADAPTATIONS. 



Passing now to other epharmonic characters of the vegetation in the 

 Ilygrophile forest formation, we encounter some very interesting mod- 

 ifications of structure in certain of the swamp trees which are believed 

 to have as their function the furnishing of air to the roots, which lie 

 in a substratum unusually poor in oxygen. The roots of certain trees 

 which grow in the water or in saturated soil in various parts of the 

 world 2 develop pneumatophores — projections which rise vertically 

 above the surface. These processes, strikingly different from most 

 roots in their negative geotropism, are believed to perform the func- 

 tion of supplying air to the roots, swamp soils being notoriously defi- 

 cient in oxygen, and with their light, spongy cortex they seem well 

 adapted to the purpose. 



In North America the "knees" of the bald cypress (Taxodium dis- 

 tichum) are a well-known example of this habit (PI. LXXIII). They 

 are most conspicuously developed when the tree is growing in a water- 

 covered soil. In the great morasses of the Lower Mississippi and its 

 tributaries these conical outgrowths often rise to a height of 2 meters 

 (about G feet) from the roots on which they originate. In Virginia, 

 however, they are never so tall. Shaler 3 believes that the largest 

 cypress knees in the Dismal Swamp do not exceed '.) decimeters (3 feet) 

 in height,, measured from their base on the root proper. My own 

 observations indicated that the knees rise usually about 3 decimeters 

 (1 foot), but sometimes or 8 decimeters (2 to 2| feet) above the 



1 See p. 390. 



'' Compare Goebel. Ueber einige Eigentumlichkeiten der siidasiatischen Strand - 

 vegetation; Pflanzenbiolog. Schilderungen, Theil t. The occurrence on the roots 

 of pneumatophores is there noted in certain palms, in the sugar cane, in a species 

 of Jussiaea, and especially in two trees of the mangrove formation, Sonneratia 

 aeida, and an Avicennia (pp. 139 to 144). In his paper on " Wasserpflanzen," 

 Biolog. Schild. Theil 2, pp. 256 to 259, Goebel notes the presence of pneumatophores 

 in the case of some semiaqnatic plants. 



;i Ann. Rep. U. S. Geol. Surv., vol. 10, p. 323. 



