GOODLATTE ." ThE ANATOMY OF PaROSELA SPINOSA 577 



from the third year on. The crystals are increasingly numerous 

 as the stem develops. Lignification begins, naturally, in the ducts 

 and tracheids ev^en of the very young stem, and proceeds as the 

 plant grows older, through the walls of the primary stereome, the 

 pith, the inner walls of the parenchyma cells lying next the prim- 

 ary bast bundles, the walls of the secondary stereome, of many 

 scattered palisade cells, and finally occurs in the cells of the bark 

 when this is formed in the sixth year. As the lignified areas in- 



r 



crease, the areas containing walls of unmodified cellulose become 

 reduced to those of the leptome, cambium, phellogen, and whatever 

 remains of the palisade tissue. Starch is found from the first to the 

 third year in the contents of the palisade, pith, parenchyma, and 

 medullary ray cells. After this it disappears as the stem becomes 

 less and less an assimilating organ. Proteids, likewise, occur in 

 conjunction with the green tissue and decrease with it. 



In the parts of the stem which become modified into spines 

 the process of lignification takes place much more quickly than in 

 the stem that continues to grow. In place of parenchyma cells a 

 stereome ring fills the spaces between the hadrome masses of the 

 bundles. This is especially noticeable as a point of difference 

 between a very young spine and a ver}^ young shoot (figures i 

 and 2). After this rapid development of tissue that serves to give 

 strength and rigidity, the growth of a spine is limited to simple 

 thickening and enlarging, for there is no secondary thickening. 



In the growing stem secondary thickening takes place accord- 

 ing to the ordinary dicotyledonous type from a cambial ring. The 

 sieve-tubes are comparatively short and wide. The vessels dififer 

 greatly as to size, but all have simple, oval pores, as is character- 

 istic of the family. The walls of the smaller tracheids are spirally 

 thickened. The medullary rays, as usual In the greater number 

 of Papilionaceae, consist of about three cell-rows, somewhat in- 

 creased toward the outer part in a wedge shape. 



Solereder * quotes Saupe to the effect that the medullary ray 

 cells of the Galegeae are small and round in tangential section. 

 In this case, however, they prove to be oblong and almost rectan- 

 gular. The wood-prosenchyma cells, where they come into con- 

 tact with the ducts, have, like them, simple oval pores. There 



* Loc. ciL 310. 



