December. 1910. 



KNOWLEDGE. 



481 



the oxidations go no further than lactic acid, which is then 

 changed into alcohol and carbon dioxide. In the absence of 

 zymase the o.xidation is carried further, producing various 

 other acids. 



PRUSSIC .ACID .\S PL.AXT FOOD.— It has been known 

 for some years that certain plants accumulate relatively large 

 quantities of free hydrocyanic acid in their tissues. The late 

 Dr. Treub iAiin. jard. Bot. Buitcnzorg. 1910) found that 

 the amount of this acid in Sorghuin plants increases during 

 the day, not owing to the direct action of light, but in 

 proportion to the formation of the products of carbon 

 assimilation. He had already shown, from investigations on 

 Paiigitim cdtilc and Phascohis Iniiatus. that light has no 

 share in the formation of hydrocyanic acid, except as it 

 favours photosynthesis (carbon assimilation), and much the 

 same results were obtained with Primus javanica. In 

 several other plants also Treub found a direct proportion 

 between the formation of this acid and the function of 

 chlorophyll, and this was confirmed by experiments with 

 variegated leaves. The amount of acid is usually greatest in 

 the young leaves, gradually diminishing as the leaves grow 

 older. With Sorghum, young leaves in a dry season or on 

 dry soil contain so much acid as to be dangerous as food for 

 cattle. Leaves about to fall contain very little acid, as a 

 rule, though Guignard has shown that fallen leaves of Elder 

 contain large amounts of the acid. The prussic acid is 

 probabl}- the first simple organic product of the assimilation of 

 nitrogen ; the amount of acid in plants watered with solution 

 of nitrate of soda or potash increased or decreased in pro- 

 portion to the amount of nitrate used. Treub agrees with 

 Ravenna and Peli's view that nitrates and carbohydrates are 

 necessary to the formation of this acid in plants. In most 

 cases the acid probably does not occur as such, but in the 

 form of a compound (glucoside), from which it can readily be 

 liberated by an enzyme, or by boiling water. 



HELIOTROPISM. — Some interesting papers have been 

 published recently upon hcliotropism — the curvature move- 

 ments by which plant organs respond to the stimulus of light. 



Guttenberg [Ber. deutsch. bot, Ges.. 1910; Jahrb. fiir 

 wiss. Bot., 1910) has shown that, contrary to the views of 

 most writers, the effect of the geotropic (gravitation) stinuilus 

 is not annulled by the light stimulus, but that it is possible to 

 choose light of such intensity that when it strikes from below 

 a seedling shoot placed horizontally, its joint action with 

 gravity will lead to the shoot permanently growing in the 

 horizontal position. Similarly, when a shoot is growing 

 vertically, light of the right intensity will, when directed 

 horizontally on the shoot, cause it to grow at an angle of 

 forty-five degrees. The intensity of light required to com- 

 pensate gravity, as in these two experiments, varies greatly 

 with different plants ; sometimes it is greater than one candle 

 power, but as a rule it is only a fraction of a candle power. 



Froschel LAnz. Akad. Wiss. WIeii. 1909) re-investigated 

 the '■ presentation time " for heHotropism. that is, the 

 minimum period for which a seedling grown in darkness 

 requires to be exposed to light in order that, when the plant is 

 darkened again, a distinct curvature may take place. The 

 extraordinary sensitiveness of the heliotropic reaction is shown 

 by the fact that an exposure of 0'0005 second to direct 

 sunlight, or to the light of a mercurv- lamp, is sufficient to 

 induce curvature in seedlings of Oats. In weak, diffused light 

 an exposure of one-fortieth of .a second is necessary. 



Haberlandt (Jahrb. fiir u'/ss. Bot.. 1910) has replied to 

 various critics of his theory of light perception in lea\es. It 

 has long been known that in seedlings the tip of the shoot is 

 the sensitive region as regards light. Ordinary leaves place 

 themselves at right angles to the light falling on them, the act 

 of perception being evidently performed by the leaf-blade, 

 while the necessary movement is made by the stalk. If sense 

 organs for light exist they must occur in the blade, and we 

 might expect such organs to lie on the surface. On this 

 reasoning, and on the results of his experiments and anatomical 

 researches, Haberlandt founded his theory that the epidermal 

 cells act as lenses to collect and condense light upon the under- 



lying green cells. To these cells of the epidermis, which have 

 curved dome-like outer walls, he applies the term " ocelli," 

 since they resemble a simple type of eye — a lens and a sensitive 

 protoplasm layer on which the light may be focussed. Haberlandt 

 admits that the epidermis cells on the lower side of various 

 leaves are able to condense light, but he points out that the 

 lens-like form of the cells is always more pronounced in the 

 upper epidermis. He points out various weaknesses in the 

 criticisms of Wager and other writers, and it must be admitted 

 that Haberlandt has greatly strengthened his position. The 

 suggestion, made by Wager, that the chloroplasts may serve as 

 organs for light perception is easily disposed of, for the leaves 

 of some variegated plants may be devoid of chlorophyll, yet 

 such leaves respond in the usual manner to light. 



Jensen { Ber. deutsch. bot. Ges.. 1910) has investigated the 

 conduction of the stimulus of one-sided illumination from the 

 tip of an Oat seedling, where the stimulus is perceived, to the 

 lower portion where (even when this basal part is darkened) 

 the curvature takes place. Rothert had shown that the 

 stimulus is conducted when the veins are cut, and that a 

 horizontal incision on one side — whether towards, away from, 

 or on one flank in reference to the one-sided illumination — 

 still allows conduction. Fitting showed that when the incision 

 was away from the light and a mica plate inserted, 

 no conduction occurred. The insertion of a slice of bamboo 

 in the same position did not prevent conduction ; in this case 

 there was, of course, continuity across the incision as regards 

 water, which could pass through the slice of rattan. When 

 the mica plate was inserted in an incision on the lighted side, 

 conduction was not hindered. Jensen states that with the 

 incision away from the light, no conduction ever occurs in 

 dry air or in water. He concludes that under fa\ourable 

 conditions the stimulus can be conducted across the wound, 

 while under unfavourable conditions it cannot. In saturated 

 air the stinuilus was conducted from the tip to the (darkened) 

 base of the seedling, even after the tip (nearly half an inch) 

 had been cut off. 



It was shown by Berthold, in 1SS2, that certain seaweeds 

 are positively heliotropic U.e. grow towards the light) in weak 

 light, but negatively heliotropic in stronger light. This 

 discovery was confirmed by Oltmanns (1897) in the case of a 

 mould-fungus iPhycomyces), the erect spore-producing 

 threads of which varied in their heliotropic response according 

 to the distance from the source of light — an arc lamp ; at 

 SO centimetres the stalks curved towards the lamp, at 20 to 

 30 cm. they curved away from it, and at an intermediate 

 distance, of course, they remained erect. Linsbauer and 

 Vonk {Ber. deutsch. bot. Ges., 1909) have recently shown 

 that the same applies to the roots of Radish and Mustard 

 seedlings. As is well known, the radicles of these plants 

 normally show very marked curvature away from the light — 

 hence they are often used in botanical teaching to demonstrate 

 negative heliotropism. It appears, however, that in feeble 

 light the roots of these seedlings show positive heliotropism, 

 curving towards the source of light. It is known that some 

 roots grow towards light under normal conditions, e.g.. the 

 roots gisen out by an onion bulb when grown in water in a 

 " hvacinth glass " placed near a window, but it is probable 

 that these and other cases of differing heliotropic responses in 

 plants will all be found to obey the same general rule — that 

 the nature of the response made by any organ simply depends 

 upon the intensity of the light. 



CHLOROPHYLL. — .Among various recent observations 

 on chlorophyll, two small but interesting items may be 

 mentioned. 



Podiapolsky \Biol. Zeitschr., 1910) has shown that chloro- 

 phyll is present in the wings of locusts and in the skin of green 

 frogs ; in each case an alcoholic extract gave, on examination 

 with the spectroscope, the characteristic absorption-bands of 

 chlorophyll — in the red region between B andC. There is no 

 question here of green organisms living in symbiosis with the 

 animal, and apparently the chlorophyll is produced in the 

 animal itself. The writer promises a detailed investigation of 

 the matter, with a view to settling such questions as : How 

 is this chlorophyll produced ? Is it concerned in the nutrition 



