WILSON: ANATOMY OF ALFALFA. 296 



fascicular tissue composed of parenchymatous sclerenchyma, 

 which is connected externally with the groups of hard bast be- 

 longing to the vascular bundles. He also mentions that in 

 some species of Medicago the interfascicular tissue consists of 

 sclerenchymatous prosenchyma and parenchyma. In alfalfa 

 we have the prosenchyma distributed into the groups of fibers 

 discussed above, and these groups, as well as the bundles as a 

 whole, connected by sclerenchymatous parenchyma. (Fig. 2.) 



The essential elements of the vascular bundles, the water 

 tubes and the soft bast, are arranged collaterally. The phloem 

 (fig. 9, 0, and fig. 7, p) is made up of sieve tubes (fig. 18) and 

 phloem pai'enchyma, together constituting the soft bast. This 

 region is rich in nutritive value. The sieve tubes are rows of 

 very narrow, elongated cells (fig. 18), with end-wall sieve 

 plates (fig. 18, h) of the usual type. The tubes are bound to- 

 gether by a network of thin-walled phloem parenchjona cells. 

 According to Moeller, short articulations and the exclusive oc- 

 currence of simple sieve plates are characteristic of the sieve 

 tubes. 



The xylem of alfalfa forms an almost continuous cylinder 

 around the mature stem. Between the bundles the xylem por- 

 tions are connected by lignified parenchyma cells (fig. 7, r). 

 The water tubes and tracheids are generally about three rows 

 deep in the bundles. The vessels are frequently bound together 

 by radial rows of wood parenchyma cells (fig. 7, y) or by wood 

 fibers. The spiral thickening has a marked border in some of 

 the tubes. 



About half as numerous as the spirally thickened tubes 

 (fig. 9, x) are the rows of pitted tracheids (fig. 9, w), which 

 are found next to the phloem. The pits are typically bordered 

 (fig. 9, iv). These occur sometimes in straight rows, some- 

 times in spiral arrangement. They are most numerous in the 

 vessel wall where it is in~ contact with ray or wood paren- 

 chyma. When arranged in straight rows there are usually 

 three rows down the tracheid, the number of pits to a mm. in 

 length being about 900. In some of the pits peculiarities of 

 structure were observed (fig. 13, t) . These may be the same 

 as the sieve-pit-like structures of bordered pits found by John- 

 son in Medicago, Lupinus, and Melilotus, and mentioned by 

 Solereder. 



