Delay argument. The assumption is that, since perception of pain is delayed, a different set of neurons, with slower conduction times, is required. There is good evidence for a progressively increasing latency of action potentials in blocked nerves. Clark, Hughes, and Gasser (9) found slight slowing of conduction rates apparent within 15 minutes. Records of action potentials in the human ulnar nerve show progressive increase in latency after inflation of the pressure cuff (10). There was no discontinuity in the curve as would be expected if the composition of the group of active neurons had changed. Similar delays in perception occur in the other cutaneous senses also (6, 7). I would assume the perceptual delay to be due to the increasing latency of action potentials in neurons subjected to pathological conditions, and possibly also to synaptic delays occasioned by reduction in number of afferent impulses reaching the central nervous system.Reliability of ischemia-nerve block data. The assumption is that such data are reliable and give reliable indices of conduction times of fibers. It is clear that the results of such experiments on human subjects are variable. If a large number of subjects is used and if the results are treated statistically (6, 7), the order of loss is seldom significant. Landau and Bishop themselves (4) found procaine blocks to be "inconclusive" because prick and deep pain disappeared together-that is, because the sensory results did not bear out the results of action-potential studies. Whereas touch may usually fail before pain in compression of a limb, the difference is not sufficiently dramatic to enable one to distinguish between small delta fibers and C fibers. There is evidence that the survival time of fibers under compression block is influenced by factors other than conduction rates. Frankenhauser (11), who dealt with touch fibers of different types, found that slowly adapting touch receptors in the rabbit were blocked later than hair touch fibers in spite of the fact that their conduction rates completely overlap those of the latter. He concluded that the fibers themselves have properties which are not predictable from observation of the impulses. In man, skin touch and hair touch also have different survival times (12), and in some areas hair touch survives pain (6).There are some interesting results which suggest that the somatic sensory apparatus is much more complex than the current popular notions would have it. Between giving up all specificity, as Sinclair (13) does, and being bound to one or even two specific pain modali-Delay argument. The assumption is

Removal of atmospheric carbon dioxide enhanced penetration of seedling wheat by stem rust Puccinia graminis in light and dark but did not materially affect penetration by leaf rust P. recondita. A concentration of 5 percent CO(2) nearly suppressed penetration by P. graminis but not by P. recondita. Thus light may promote penetration by P. graminis through photosynthetic reduction of CO(2) within the leaf and P. recondita may penetrate independently of light because it is relatively insensitive to the effects of CO(2).

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Synopsis Hessian fly resistant wheats infested at first leaf stage, unlike susceptible wheats, undergo little stunting of subsequently developed leaves. Larvae migrated normally downward, but were constantly carried upward by the elongating second leaf. Four days after hatching, most larvae had died in the sheath column and were subsequently carried upward and above the enclosing leaf sheath. Normal growth of resistant leaves appears detrimental to larvae, but its relationship to the initial lethal effect is not understood.

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