128 Guayule. 



ABBREVIATIONS USED IN PLATES 22 TO 39. 



pc. pericambium. cot. cotyledon. 



end. endodermis. cor. cortex. 



ex. exodermis. r.c. resin-canal. 



/,,/ 2 . primary and secondary leptome. ped. peduncle. 



hi,h 2 . primary and secondary hadrome. pr. parenchyma-ray (or cells of this) . 



s. stereome. lib. libriform. 



m.c.t. median cotyledonary trace. /. lacuna. 



l.c.t. lateral cotyledonary trace. Ip. leptome. 



c.t. cauline traces. pd. periderm. 



c.t.l. cauline traces, first primordial leaf. 



DESCRIPTION OF PLATE 22. 



1-5. Development of endodermal canals of root. Fig. 3, root 8 mm. diameter. 



6. Lateral growth of endodermal cells and intercalated walls, with Caspary's 



band. Root 0.6 mm. in diameter. 



7. Cortex (with exodermis) of a tap-root 0.46 mm. in diameter. First peri- 



dermal divisions in the hypodermal cells. 



8. The reduction of cortical cells into cork has reached the endodermis. 



9. Portion of a tap-root showing early cambium divisions. 



10. The same in a somewhat older root to show development of intercalated 



mestome strand (h 2 , 1 2 ) at outer edge of primary hadrome plate (/t t ). 



11. A still later stage showing closure of secondary hadrome about the parenchyma 



island by uniting with intercalated mestome strands. The separation of 

 these from the primary plate is due to disturbance by the growth of a second- 

 ary root. 



12. Primary leptome in contact with the endodermis, in the hypocotyl. 



13. A secondary cortical canal arising in the phloem. The meatus is just at this 



moment appearing. 



14. Details of secondary leptome, showing the relation of the resin-canal to the 



remaining elements. Root 4 mm. in diameter. 



15. Pores in walls of endodermal and cortical cells (cf. plate 41, fig. 2). 



1 6. Pericambium in a root after primary stereome has begun to develop. 



