614 BOWMAN— ECOLOGY AND 



centimeters behind the cap. The concUictive bundle cyhnder is com- 

 posed of about 30 or 40 alternating strands of xylem and phloem 

 tissue. As Warming has also shown, however, a most unusual de- 

 parture is made from this regular root arrangement in that there are 

 often more than one phloem strand between two xylem patches, as 

 seen in transverse section. This is supposed to occur by the splitting 

 of strands. The phloem strands contain both sieve tubes and phloem 

 parenchyma. The xylem in its earliest state, i. e., protoxylem, has 

 very few spiral trachess, just behind this externally is a small group 

 of soft bast elements, the tracheae being surrounded by a scleren- 

 chyma ring or sheath. In this development, the method of growth 

 is centrifugal. Beyond this group of phloem elements is the xylem 

 strand and this has the peculiar structure of a double bundle, but 

 both are enclosed in one sclerenchyma sheath. What causes this 

 splitting in the xylem it is not possible to say. Among the xylem 

 elements are scattered large pitted and scalariform vessels. The 

 phloem is now very well developed. 



The pith of the root is of large thin-walled cells, typical medul- 

 lary tissue with intercellular spaces in which lie many trichoblasts. 

 The pith also contains tannin cells. 



The Stem. 



The twigs and branches of Rhisophora show little that is pecu- 

 liar in the general arrangement of the structures. In the wood, 

 however, there are prosenchymatic vessels which are pitted and also 

 there are some vessels which have ladder-like perforations. These 

 appear as holes with transverse bars across which in most instances 

 number about four or five. The medullary rays are rather broad 

 and where the bundle vessels come in contact with the ray tissue 

 the walls of the former are pitted. 



The cork formation, according to Solereder and IMoller,'^'' is 

 superficial, and of the spongy type. In the pericycle there is a dense 

 ring of sclerenchyma, which makes the twigs very difificult to cut. 



''^ Moller, J., " Hol/.anatomie," Dcutschr. JViciicr Akad., p. 103, 1876. 



