146 Guayule. 



DESCRIPTION OF PLATE 31. 



1 . Transverse section through upper part of petiole of a well-matured cotyledon, 



in which one sees the ends of the resin-canals, rc lt rc 2 . Above this level they 

 end blindly. 



2. A section of one of the resin-canals (rc^ in fig. i, somewhat nearer base of same 



cotyledon. 



3. Transverse section 3 mm. from apex of stem of a field plant, co, cork. Slow 



growth . 



4. Transverse section through a stem in rapid growth 2 mm. below apex. The 



five medullary canals are established according to a 2/5 phyllotaxy. 



5. A section through a stem apex above that of fig. 4, in which the order of de- 



velopment of the cortical canals is seen to relate to that of the leaves. 



6. Inner part of hadrome bundle of stem, showing cells which become stereids. 



7. The same, in which the stereids are of maximum size and their walls partially 



thickened. (Figs. 6 and 7 are drawn to the same scale.) 



8. Enlargement of stereid elements in leptome previous to thickening of walls. 



9. The leptome in which the primordial cells which become stereids, st, are seen. 



10. A very young medullary resin-canal in the secretory cells of which are seen 



relatively large globules of rubber. Minute ones appear in adjacent cortical 

 cells. 



1 1 . One of these cortical cells on a larger scale, to show the rubber granules more 



exactly. 



12. Secretory cell of medullary resin-canal after periclinal divisions, showing gran- 



ules of rubber. 



13. The schizogenous origin of the medullary canal. 



14. Peridermal divisions in the collenchyma. 



