Gibberellic acid and sucrose play significant roles in the increases in invertase and growth in Avena stem segments. About 80% of invertase is readily solubilized, whereas the rest is in the cell wall fraction. The levels of both types of invertase change in a similar manner in the response to gibberellic acid and sucrose treatment. The work described here was carried out with only the soluble enzyme. In response to a treatment, the level of invertase activity typically follows a pattern of increase followed bv decrease; the increase in activity is approximately correlated with the active growth phase, whereas the decrease in activity is initiated when growth of the segments slows. A continuous supply of gibberellic acid retards the decline of enzyme activity. When gibberellic acid was pulsed to the segments treated with or without sucrose, the level of invertase activity increased at least twice as high in the presence of sucrose as in its absence, but the lag period is longer with sucrose present. Cycloheximide treatments effectively abolish the gibberellic acid-promoted growth, and the level of enzyme activity drops rapidly. Decay of invertase activity in response to cycloheximide treatment occurs regardless of gibberellic acid or sucrose treatment or both, and it is generally faster when the inhibitor is administered at the peak of enzyme induction than when given at its rising phase. Pulses with sucrose, glucose, fructose, or glucose + fructose elevate the level of invertase significantly with a lag of about 5 to 10 hours. The increase in invertase activity elicited by a sucrose pulse is about one-third that caused by a gibberellic acid pulse given at a comparable time during mid-phase of enzyme induction, and the lag before the enzyme activity increases is nearly twice as long for sucrose as for gibberellic acid. Moreover, the gibberellic acid pulse results in about three times more growth than the sucrose pulse. Our studies support the view that gibberellic acid, as well as substrate (sucrose) and end products (glucose and fructose), play a significant role in regulating invertase levels in Avena stem tissue, and that such regulation provides a mechaniism for increasing the level of soluble saccharides needed for gibberellic acid-promoted growth.

An electron microprobe (EMP) analysis of silica (SiO,) deposition in the epidermis of developing internodes of the perennial scouring rush (Equisetum hyemale var. affine) indicates that SiO, is first detected in the stomata1 apparatus beginning with internode 3, then the epidermal papillae (internode 8), and finally in radial cell walls of the long epidermal cells (internode 10). This process is initiated in the intercalary growth regions at the bases of the elongating internodes. The deposition of SiO, in long epidermal cell walls occurs after internodal extension has ceased and should therefore be considered as one of the final stages in internodal differentiation that involves strengthening the cellulosic framework of the cell wall. EMP measurements indicate that SiO, in stomata is equivalent to 30% of a pure SiO, standard and that SiO, in the radial walls of long epidermal cells averages twice that measured on the tangential walls of these same cells. This study supports the view that silicification plays a major role in strengthening the developing perennial scouring rush internodal system and that regulation of this process in this and other species of Eguisetum, whose SiO 2 deposition patterns are markedly different, deserves further study.

The recovery from "lodging," or bending over, by shoots of 42-day-old Avena sativa plants is controlled primarily by a negatively geotropic differential growth of the lower halves of the p-I node-pulvinus and the base of the p-I internode, relative to the upper halves. Although geostimulation causes a significant reduction in p-1 internode length, dry matter accumulation in the p-i node-pulvinus is increased, apparently at the expense of the sheath. Recovery to an angle of 300 is associated with changes in endogenous gibberellin-like substances (GAs), and in differential metabolism of applied I3HIGA4 (1.4 Curie per millimole). Although geostimulation depressed total GAs (relative to upright plant parts) to 0.40 and 0.13 for node-pulvini and sheaths, respectively, it increased them 2-fold for internodes. Within the plant part geostimulation increased GAs (relative to upper halves) 29-and 7-fold in lower halves of node-pulvini and internodes, respectively, but reduced GAs to 0.3 in lower halves of sheaths. At age 42 days a GA4/7-like (nonpolar) substance predominates, with lesser amounts of a GA3-llke (polar) substance. Native GAs of Avena include GA3, GA4, and GA7. Geostimulation enhanced the ratio of nonpolar to polar GAs for both halves of internodes, but tended to depress it for sheaths and nodepulvini.The disposition and metabolism of applied 13HIGA4 confirmed the above trends for endogenous GAs regarding localization (e.g. up to 2-fold increases in 13HIGA4 and acidic 3H-metabolites in the lower halves, relative to upper halves). Also, metabolism into highly water-soluble 3H-metabolites (biologically jnactive conjugates?) was greater (up to 1.8-fold) in upper than in lower halves. The end result of such metabolic trends would be to reduce acidic (biologically active?) GAs in the upper half, while retaining them for a longer time in the lower half.Geotropically stimulated Avena shoots thus increase, within 24 hours, the levels of acidic GAs in the lower halves of the p-1 node-pulvinus and p-I internode, the two plant parts responsible for the geostimulated growth.

Using scanning electron microscopy, we determined the kinds and distribution of epidermal cell types in Avella inflorescence bracts (glume, lemma, and palea). Electron microprobe analysis of silica deposition in these epidermal cells showed that silica cells constitute one of the important deposition sites. Probe ratio data indicate that the silica deposited is 74 % pure. Significant amounts of silica also become deposited in the trichomes and lesser amounts in the walls of long epidermal cells. None could be detected in the stomata. The possible functional significance of silica deposition in epidermal cells of these bracts is discussed.

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