690 MINNESOTA BOTANICAL STUDIES. 



Oenothera humifusa. — 18. Surface view of epidermis. 19. Sclerotic pith 

 cells with unpitted walls. 20. Epidermal and sub-epidermal tissues of 

 young stem. 21 and 22 Emergences. 23. Portion of the common wall 

 between two sclerotic pith cells; intercellular spaces. 



Plate XXXVII. 



Oenothera humifusa. — 24. Sclerotic cells from the pith, showing lamel- 

 lated structure and pitted walls. 25 and 26. Epidermal hairs. 



Oenothera rhombipetala.—21. Intra-xylar phloem island in the secondary 

 wood. 28. Transverse section of a portion of the secondary wood between 

 two medullary rays. 29. An island in the secondary wood seen in longi- 

 tudinal section. 30 and 31. Trichomes. 



Onagra biennis — 32. Extra-xylar tissue, transverse section. 33. Dia- 

 grammatic cross section of portion of old stem; e epidermis, c cortex, s 

 stereome bundles in the outer portion of the pericycle, k cork, i p inner 

 pericylic area, ph normal phloem, i islands in the secondary xylem, v ves- 

 sels of secondary xylem, w primary xylem, m medullary phloem groups, 

 p pith. 34. Portion of a longitudinal section through the secondary xylem, 

 showing a large pitted vessel. 35. Trichomes. 36. Reticulated cells of 

 medullary rays. 37. Spiral and pitted tracheae of the xylem. 38. Epi- 

 dermis, surface view. 39. Primary and secondary xylem. 40. Part of 

 section of young stem;p parenchyma of the cortex, ed endodermis, s ste- 

 reome bundles, k developing cork. 41. Isolated cells of the pericylic gte- 

 reome. 42. Pith parenchyma in longitudinal section. 



Plate XXXVIII. 



Onagra biennis.— 43. Hypocotyl of a seedling thirty-eight days old, cross 

 section; e epidermis, p parenchyma of the cortex, ed endodermis, 1 1 sieve 

 tubes, c cambium, x xylem, pi pith. 44. Medullary phloem group sur- 

 rounded by parenchyma. 45. Extra-xylar portion of an old stem, longi- 

 tudinal section. The periphery of the section is at the left of the draw- 

 ing where the cork of characteristic structure is seen. 46. Transition 

 from xylem to phloem; a medullary ray is shown terminating in a crystal 

 sac. 47. Intra-xylar phloem islands. 48. Cross section of the hypocotyl 

 of a seedling eight days old. The epidermis, cortex and endodermis are 

 easily distinguished. The various tissues of the stele show but slight 

 differentation. 49. Cross section of young stem previous to cork forma- 

 tions epidermis, c collenchyma, p parenchyma of the cortex, ed endoder- 

 mis, s stereome bundles, t sieve tubes, m medullary ray, c cambium, x 

 xylem. 



Figures 43 and 48 are magnified about 600 diameters: the other figures 

 represent a magnification of 200 to 300 diameters. 



