PHYSIOLOGY OF THE RED MANGROVE. 611 



which are subterranean or submarine, buried in the mud, and which 

 have assumed a purely absorptive role, are called the absorptive 

 roots in this paper. Van Tieghem*^^ has described and figured the 

 root of the Rhisophora, and shows especially the development of 

 these secondary roots. He says: "An arch of the pericycle of the 

 width of three cells in the external layer and corresponding with a 

 wood bundle, increases and cuts off two rows of cells, but espe- 

 cially does the external layer increase, and it is this alone, by two 

 tangential wall formations, which differentiates the three regions of 

 the rootlet from the original cells. The internal row does not go 

 beyond the base of the central cylinder. The superimposed arc of 

 the endodermis dilates its elements, but not radically, and encloses 

 the developing rootlet by an absorptive pouch. In this pouch, which 

 is dilated to a great extent, the rootlet elongates rapidly the width 

 of the cortex, but remains very narrow. More slowly it then en- 

 larges at the summit and the pouch is absorbed laterally, but the 

 terminal part is left adhering like a cap as it emerges from the root." 

 This interesting process may be seen on both the hypocotyls of 

 seedlings and the origin of the dependent prop-roots from the 

 branches. These little root caps adhere for quite a long period, 

 especially in the aerating roots. If the tip of one of these pendant 

 roots is injured, there will be a division just back of the tip and the 

 geotropic growth will continue as two or three branches. These 

 branches usually push out at the lenticel with which these aerating 

 roots are well supplied. The same thing occurs on the hypocotyl 

 which also is supplied with lenticels. If the roots at the radicular 

 end are destroyed adventitious roots are put out up farther on the 

 hypocotyl, perhaps just a few centimeters below the plumule. What 

 the stimulus may be is not exactly known in this case, but in as 

 much as oxygen has been shown to be stimulating in the production 

 of root hairs in plants, it may be presumed that the supply of oxygen 

 received through the lenticel acts as a stimulus for the production 

 of the rootlet from the pericambial tissue just at the point beneath 

 the lenticel. The initial stimulus for the production of these adven- 

 ts Van Tieghem, Ph., and Douliot, H., Ann. dcs Sci. Nat. Botaniquc, Ser. 

 7, Tome 7, p. 212. 



