126 THE ELEMENTS OF VEGETABLE HISTOLOGY 



the thin walls. Medullary rays are arranged in groups 

 or bundles, each of which consists of a number of 

 individual ray cells. Upon different aspects these 

 ray groups present striking differences in appearance; 

 and one can readily determine the direction in which 

 a specimen has been sectioned by differences in the 

 appearance of ray cells. Seen in tangential sections, 

 the bundles appear to be elliptical and composed of 

 nearly circular cells (Plate 615). Upon radial view 

 the bundles appear as wide strips made up of large 

 rectangular cells (Plate 61A). Upon transverse 

 section the bundles appear as narrow strips composed 

 of one to five rows of rather long rectangular cells 

 (Plate 60) . In powdered materials the rays are usually 

 seen on radial view. The contents of medullary ray 

 cells include water, starch, crystals, resins and tannins. 

 Latex Tubes. — In certain families of plants con- 

 ducting tissues known as latex tubes are present. 

 These structures contain and transport a milky juice 

 and derive their name from this fact. Latex tubes 

 are uninterrupted tubular structures which extend 

 through the various tissues of certain plants. They 

 may attain great length, are considerably branched 

 and are non-porous (Plate 46, No. 3). The exact 

 function of this milky juice or latex is not clear. As 

 the juice is usually acrid and purgative it may be a 

 means of protection against animal attacks. The 

 latex coagulates upon exposure to air and effectu- 

 ally seals wounds in plant tissues. Digestive enzymes 

 are contained in the latex of certain plants, notably 

 Carica Papaya, which contains the proteolytic ferment, 

 papain. The latex tubes are best viewed in longi- 

 tudinal sections and appear as irregularly branching 



