On the Determination of Vascular Patterns During Tissue Differentiation in Excised Pea Roots Author(s): John G. Torrey Source: American Journal of Botany, Vol. 42, No. 2 (Feb., 1955), pp. 183-198 Published by: Botanical Society of America Stable URL: http://www.jstor.org/stable/2438468 . Accessed: 23/08/2011 15:22 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Botanical Society of America is collaborating with JSTOR to digitize, preserve and extend access to American Journal of Botany. http://www.jstor.org February,1955] TORREY-VASCULAR PATTERNS IN EXCISED ROOTS 183 throughout the United States. (3 vol. and atlas). POTONIE, H. 1889. Uber einige Carbonfarne. Jahrb. K. Second Geol. Surv. Pennsylvania Rept. Progress P. Preuss. Geol. Landesanst. 10: 21-27. p1. II-V. MAMAY,S. H. 1950. Some American Carboniferousfern RENAULT, B. 1883. Cours de botanique fossile. Vol. III. fructifications.Ann. Missouri Bot. Gard. 37: 409-476. Paris. MOORE, R. C., ET AL. (Pennsylvaniansubcommittee of the WILSON, C. L. 1953. The telome theory. Bot. Rev. 19: National Research Council committeon stratigraphy). 417-437. 1944. Correlationof Pennsylvanianformations of North ZIMMERMAN, W. 1930. Phylogenie der Pflanzen. Jena. America. Bull. Geol. Soc. America 55: 657-706, chart . 1952. Main results of the "Telome theory." The no. 6. Palaeobotanist 1 (Birbal Sahni mem. vol.) : 456-470. ON THE DETERMINATIONOF VASCULARPATTERNS DURING TISSUE DIFFERENTIATION IN EXCISED PEA ROOTS' JohnG. Torrey ONE OF THE mostcharacteristic and diagrammatic Biinning)have clearlydemonstrated the continuous patternsestablished in theseedling structure during early establishmentof the radial vascular pattern the ontogenyof the individualplant is the alternate in the root proximalto the apical initialsand the and radial arrangementof the primaryvascular uninterruptedacropetal maturation of vascular tissueswithin the primaryroot. The determination tissues. of a precise radial arrangementof alternating The experimentalstudies, limited in number, strandsof phloemand xylemtissue by longitudinal have been reviewedrecently by Esau (1954). Two differentiationoccurs during early embryogeny. major aspectsconcerning the determinationof the The vascular patternis evident in the seedling pattern of vascular differentiationhave been radicle. In many species,this radial patternwith analyzedusing experimentalmethods. Jost (1931- its characteristicnumber of vascular strandsmay 32) fromhis studieson vascular differentiationin' persistfor the life of the primaryroot. regeneratingdecapitated roots concludedthat the Prior investigationsof the originof the vascular maturevascular tissues of the root induce differ- patternin roots presenttwo general approaches, entiationof thenewly formed cells derivedfrom the each of whichmay have significancein understand- apical initials into the establishedpattern of the ing the basis of the pattern.One approachinvolves matureregion. This conceptof longitudinalinduc- detailedanatomical studies of the.root structurein tion arises from Pfeffer(1904) and Haberlandt all parts of the root system-primaryaxis as well (1913) 'who reportedthe controlof cell division as branchroots of differentsizes and ages. The and differentiationof meristematiccells by mature otherapproach has been one of experimentalmanip- differentiatedvascular tissues. Supportfor the idea ulation,usually by surgicaloperation, in an attempt of vascular tissue determinationby inductionwas to influencethe tissue differentiationand pattern givenby someearly root-culture work (e.g., Bonner formation. and Addicott,1937). Furtherindirect support for Descriptivestudies reportedextensively in the this view of differentiationhas been given by the classicalanatomical literature (van Tieghem,1870- consistentobservation in anatomicalstudies of the 71; DeBary, 1884) have been interpretedin such acropetal sequence of vascular tissue maturation works as Bower (1930) and are summarizedby in roots. Esau (1953). It has been shownthat the number of alternateand radiallyarranged xylem and phloem The opposedview holds that the determination of- strandsmay be constantin any givenplant and may the vascular patternin roots is controlledby the^ be quite characteristicof a species,genus, or even apical meristemwhich governsthe differentiatiorn family. In some plant groups the number of of the primarytissues of theroot. Thoday (1939)' vascularstrands in the primaryroot varies among has emphasizedthe self-determiningpower of the membersof the group (Wardlaw,1928) or varies root apex and has cited the evidencederived from within the root of the same individual (e.g., culturingisolated root tips in nutrientmedium. As as Heimsch,1949; Meyer, 1930). Lateral branches early 1931 Scheittererconcluded from experi- fromthe primaryroot axis oftenshow a reduced mentswith very small excised root tips which grew number of vascular strands. Recent anatomical and differentiatedin nutrientculture that the root analysesof rootontogeny (e.g., Esau, Heimsch,and meristemcontrols primary tissue differentiationin' the root. Recently,Biinning (1952) has reported' 1 Receivedfor publication June 15, 1954. thatif a 2-mm.root tip of Vicia faba is excisedand' This investigationwas supportedin partby research replacedon the shortenedstump of the same root,. grantnumber RG 2861 fromthe NationalInstitutes of it would forma union and grow,but the xylem' Health,Public Health Service. The writeris indebtedto Mr. MiltonAnken for his elementsof the two parts are not in line and only excellenttechnical assistance. laterbecome connected by differentiatingelements. 184 AMERICAN JOURNAL OF BOTANY [Vol. 42 He concludedthat the vascular pattern was already elsewhere(Torrey, 1954). The procedurecan be fixedin the 2-mm.root tip and thatdiffusible sub- summarizedas follows: Alaska pea root tips 5.0 stancesmoved fromthe maturetissues toward the mm. in lengthwere excised from sterilizedseed tip which made possible the maturationof the germinatedfor 48 hr. and weregrown in a synthetic vessel elements. nutrientmedium containing mineral salts, 4 per The secondmajor aspectof vasculartissue differ- cent sucrose and 0.5 per cent agar, plus FeCl3, lack- entiationin rootsconcerns the question of thedeter- ing added vitamins and trace elements. After minationof theradial and alternatearrangement of growth for 1 week to an average root length of xylem and phloem tissues in the primaryroot. about 50-60 mm.,the terminal0.5-mm. tips (includ- Accordingto thedescription of differentiationin the ing root cap) were excised from the control roots diarch roots of Sinapis by Buinning(1951), the under a dissecting microscope equipped with an early enlargementof the centralfuture-metaxylem ocular micrometer and were transferred to fresh elementsimmediately behind the apical initials medium in small dishes. Growth of these experi- exercisesa determininginfluence on the subsequent mental tips was dependent upon the addition of the differentiationof all the tissues of the central vitamins, thiamin and nicotinic acid, and trace cylinder.This influenceresults in the inductionof elements. Such 0.5-mm. root tips were grown for differentiationofthe radially-aligned vessel elements various periods from one to several weeks and were of the xylemand the suppressionof phloemtissue then killed in formalin-aceticacid-alcohol (FAA) except at the radial points most distantfrom the with aspiration. Root tips were dehydrated in an induced xylemtissue. Accordingto Biinning,no ethyl-butylalcohol series, embedded in "Tissue- phloemcan arise in the immediateneighborhood Mat" and sectioned on a rotary microtome at 8,u. of a xylemstrand nor in the vicinityof existing Serial sections were stained with Heidenhain's phloemtissue. Thus a radial and alternatepattern hematoxylin and safranin. Histological study of of vascular tissue must result. Biinning (1951, large numbers of roots in any one experimentwas 1952) has used stainingreactions, measurements of accomplished with roots killed in FAA and sectioned nucleolarsize, and the resultsof woundingexperi- at 40M using a carbon-dioxide freezing microtome. mentsto supporthis speculations. Sections were mounted in water and examined under In studyingthe determinationof vascular-tissue the microscope immediatelywithout staining. patternsin the root, it is essentialto analyze the The root diagrams used as illustrationswere made problemnot in termsof two-dimensionaltransverse by drawing cellular outlines directly on photo- or longitudinalroot sections,but in the light of graphic prints, removing the emlusion and com- the three-dimensionalroot structureas a whole, pleting the line drawings in ink. Both the original consideringlongitudinal as well as radial forces photomicrographand its line tracing are presented actingupon the system.The problemthen resolves to facilitateinterpretation. itselfinto two fundamentalquestions. First: is the Structure and vascular tissue differentiationin determinationof the acropetal differentiationof excised control pea roots in culture.-The organiza- primaryvascular tissues in theroot inducedby the tion