Fig. 3. Weak alleles are defective in meristem maintenance. (A-L) Primary root meristems of gnom^R5, gnom^B/E and wild-type at (A-F) day 7 and (G-L) day 15. (A,D,G,J) gnom^R5, (B,E,H,K) gnom^B/E, (C,F,I,L) wild type. (D-F,K,L) Higher magnifications of A-C,H,I, respectively. Arrowheads in A-C,H,I indicate approximate position of the onset of cell elongation. (G,J) Two examples of collapsed gnom^R5 root meristems at day 15 (arrows, vascular strands; asterisks, bloated root hairs). (M) Root lengths of gnom^B/E and gnom^R5 relative to wild type. Both genotypes are significantly different from wild-type control (P<0.001). (N) Lateral root density of gnom^B/E and gnom^R5 relative to wild type. Note that in gnom^R5, lateral roots were never observed. Both genotypes are significantly different from wild-type control (P<0.001). (O) Gravitropic growth response of gnom^B/E as compared to wild type. Seedlings grown upright were turned by 135° and re-alignment to the gravity vector was recorded after 36 hours. Each root was assigned one of twelve 30° sectors. Genotypes were significantly different (P<0.001).