422 



SCIENTIFIC NEVS^S. 



[May 4, lE 



for a day. With care and under favourable conditions, 

 large healthy plants may be grown in this way. When 

 grown to a suitable height, the plant must be removed, 

 cut up small, with scissors, dried at ioo° C, and weighed. 

 The result of the experiment may be set down in this 

 form : — 



Weight of seed ... ... ... ... — 



Of which the water, driven oif at loo" C, 

 amounted to ... ... ... ... — 



Weight of plant after — weeks' cultivation — 

 Weight of plant, dried at ioo° C. ... — 



Increase of organic substance at the expense 

 of inorganic material ... ... ... — 



Being — times the original weight. 

 (The mineral constituents may be neglected.) 

 If a plant, grown from the seed under the above con- 

 ditions be kept in the dark, it never acquires a green 

 colour. Dry and weigh it after some weeks, when its 

 weight will be found to fall considerably below that of 

 the original seed. Waste by oxidation has gone on 

 steadily, and there has been no formation of new organic 

 substance. 



Experiment 2. — Proof that green plants liberate oxygen 

 under the influence of light. 



Enclose a number of green water-weeds in a glass 

 funnel, which rests, mouth downwards, in a beaker of 

 water. Invert a test tube, completely filled with water, 

 over the funnel. Place the apparatus in sunlight. 

 Bubbles of gas arise from the plants, and collect in the 

 test tube. When the tube is sufficiently full, place the 

 thumb against its mouth, withdraw the tube from the 

 beaker, and gently turn it mouth upwards. The presence 

 of abundant oxygen can be proved by inserting a glowing 

 match. 



If it is desired to collect the oxygen rapidly and in 

 considerable bulk, the water in the beaker should be 

 aerated occasionally, but not profusely, with a carbonic- 

 acid generator. 



Experiment 3. — Proof thai green plants can only liberate 

 oxygen when supplied with carbonic acid. 

 Place green water-weeds in a flask which is fitted 

 with a cork. Pass through the cork a glass tube con- 

 nected with a V-tube, in which are fragments of pumice 

 moistened with caustic potash solution. The potash is 

 used to absorb the carbonic acid from the air, which is 

 allowed to pass freely into the flask. Expose to sunlight. 

 At first buljbles of oxygen will be seen to pass off from 

 the weeds, but with steadily diminishing rapidity. After 

 a few hours no more will form. 



Experiment 4. — Proof that starch is precipitated in green 

 leaves under the influence of sunlight, and that it dis- 

 appears in darkness. (Sachs' experiment.) 



Take a broad leaf, such as primrose or tropasolum, 

 which has been exposed to sunlight for some hours. 

 Discharge the green colour by boiling in water, and 

 afterwards in methylated alcohol. Prepare a sherry- 

 coloured solution of tincture of iodine in water. Pour 

 this into a white saucer, and place the blanched leaf in it. 



Repeat the experiment with a leaf of the same species 

 which has been kept in the dark for some hours. The 

 second plant may be kept covered with a flower-pot, 

 while the first is being exposed to light. 



The first leaf will exhibit dark purple blotches, 

 indicating the presence of starch, wherever the light has 



acted. No such coloration will appear in the second leaf. 

 Half of a leaf may be shaded by a paper envelope, while 

 the other half is freely exposed. An equally marked 

 difference in colour will be observed when the leaf is 

 blanched and iodized. Hence we learn that the assimi- 

 lating action of the light is not communicated, even to 

 neighbouring parts of the same leaf, and that the 

 precipitated starch remains in the cells where it was 

 formed. 



Experiment 5. — Proof that starch is precipitated in green 

 leaves only when carbonic acid is present. 



Place a bell-glass in a dish containing potash solution. 

 Within the bell-glass, a green plant, e.g., a primrose in a 

 pot, is to be placed, care being taken that the pot does 

 not touch the potash solution. Expose the whole appa- 

 ratus to sunlight, and after some hours test a leaf for 

 starch, as in Experiment 4. No purple patches will 

 appear. 



This experiment is sometimes modified by exposing 

 half of a leaf to sunlight in the presence of carbonic acid, 

 while the other half, still attached, is kept in an atmo- 

 sphere devoid of carbonic acid. Starch is precipitated 

 in the first half but none in the second. (Moll's experi- 

 ment.) 



Experiment 6. — Proof that green plants form proteids 

 (organic nitrogen-compounds) out of inorganic 

 nitrogen-compounds. 



Seeds of maize are germinated, and grown by the 

 method described under Experiment i. If the nitrate is 

 left out, the plant is starved, and attains only a very small 

 size, never forming any new proteids, but merely using 

 up those contained in the seed. Incidentally, this 

 experiment shows that the plant is unable to use as food 

 the abundant free nitrogen of the air. 



By a variation of the same experiment, it can be shown 

 that potassium is necessary to active growth. If a 

 sodium-salt be substituted, the vigour and size of the 

 plant are very materially affected. 



Water-culture is the chief resource for investigation of 

 the processes of plant-nutrition, and the student who 

 intends to work seriously at vegetable physiology will 

 find an ample reward for any time and pains spent upon 

 this somewhat difficult mode of experiment. 



■ ■^^!^>^*tf-^ 



^eiiiehjs!> 



Geology, Chemical, Physical, and Stratigraphical. By 

 Joseph Prestwich, M.A., F.R.S., F.G.S. Vol. ii. : 

 Stratigraphical and Physical. Oxford : Clarendon 

 Press. 

 This volume, though nominally a continuation of the 

 author's previous work, is quite complete in itself as a 

 geological history and record of biological evolution from 

 the time when life first made its appearance on the 

 earth's crust. The great physical changes that have 

 taken place from the Archaean to Post-glacial periods, 

 together with the development and successive modifica- 

 tions of life, are carefully detailed. Firstly, as represented 

 in this country, and secondly, at the end of each geologi- 

 cal period, we have the corresponding systems of Europe 

 and other parts of the world sufficiently identified and 

 enlarged upon to enable the reader to enter into the 

 wider consideration of stratigraphical geology, should he 

 so desire. At the end of each of the greater geological 



