PHYSIOLOGY OF THE RED MANGROVE. 627 



often abundantly branched and of a form which varies according 

 to the special place of their occurrence. These usually occur in dis- 

 similar lacunar tissue with their branches pushed into its inter- 

 stices. In as much as these project like many-branched hairs into 

 wide air-containing spaces (as Rhizophora) . . . and also occur in 

 many tough leathery foliage leaves . . . they appear to serve as a 

 strengthening apparatus for the tissue. De Bary further mentions 

 their occurrence in the pith and cortex of Rhizophora (p. 220) but 

 has overlooked them in other parts. He regards them as being 

 closely related to sclerenchyma fibers and only differ from them in 

 shape and distribution. 



The tannin cells do not seem to have received so much attention 

 from histologists as the idioblasts. Most investigators on Rhizo- 

 phora have mentioned the occurrence of tannic acid in large quanti- 

 ties, but few have remarked on the localization of this substance. 

 The large cells of the root-cortex, both of the aerial prop-roots and 

 to some extent of the absorptive roots, are filled with large, rather 

 polygonal cells, which contain tannin. The tannin in the cells ap- 

 pears as tiny brown granular masses, which stain a dense black 

 when special preparations are made of tissues stained with copper 

 acetate or ferric chloride. The pericycle region of the soft absorp- 

 tive roots contains the most in the subterranean roots which perhaps 

 have the least of any organ in the plant. In the leaves, as seen in 

 Fig. 4, PL IV., the large special tannin cells are the first two layers 

 of the hypodermis, just beneath the epidermis. The pericarp of 

 the fruit and even the young embryo also show notable qualities of 

 tannin in specially prepared material. The role played by tannin 

 in the economy of Rhizophora will be discussed in the next chapter. 

 The largest amount is found in the cortex of the stems and aerial 

 roots. 



Physiology. 



The physiological relation of Rhizophora to its various media of 

 growth is perhaps the main subject of consideration in this paper. 

 The idea of work on the physiology, as expressed by transpiration 

 and absorption, had its inception in the interest aroused by the ap- 

 parent ability of these trees to grow almost equally well in either 



