Makch 1, 1900.] 



KNOWLEDGE. 



close tlio opening between them. Sometimes stomata 

 are found on the uppei* sido of the leaf ; in most cases, 

 however, they are more abundant on the lower, and, as 

 a rule, aie quite absent from the upper. 



Fig. 1. — A Surface View of a portion of an Iris Leaf showing the 

 Stomata. s, a Stomata; o, the Guard-cells surrounding it. x 100. 



They are found in great numbers, as the following 

 figures show. The Poeony leaf has none on its upper 

 side, while on the lower there have been counted 13,790 

 per square inch; in the leaf of the Cheixy laurel 

 (Prunus Laurocerasus) there are none on the upper sido, 

 but 90,000 per square inch on the lower. It seems 

 natiu'al to suppose that the gases of the atmosphere 

 enter the leaf by way of the stomata, and such is indeed 

 thfe case, although it is only within the last five years 

 that this fact has become known. Previously it was 

 always stated that Carbon dioxide entered through the 

 walls of the close-fitting cells of the upper side. A 

 complete proof of the fact that it enters by the stomata 

 requires a very complicated piece of apparatus. || A 

 simple demonstration is, however, easily made. A plant 

 whose leaves have all their stomata on the lower side — 

 e.g., the Nasturtium mentioned above, or Syringa 

 (Philadelphus coronarius) — is placed in the dark until 

 all its starch has disappeared. A thick layer of 

 vaseline is then smeared over half of the lower surface 

 of one or more leaves. In this way the stomata of the 

 smeared half of the leaf are closed so that no air can 

 enter them. The plant is now placed in the light and 

 allowed to remain there for some hours. The 

 " vaselined " leaves are afterwards removed and tested 

 with Iodine in the usual manner. Starch is found 

 only in those portions of the leaves whose stomata were 

 not closed by vaseline. No assimilation then takes 

 place where the stomata are blocked against the en- 

 trance of Carbon dioxide, which shows that they are 

 literally the " mouths " by way of which the plant 

 receives its Carbon. 



The stomata open into passages which wind about 

 among the cells of the leaf. The air which enters passes 

 roimd and between these cells, giving up to them 

 Carbon dioxide and receiving from them Oxygen and 

 the vapour of the water which passes out through the 

 stomata. Inside the cells are protoplasm and cell sap, 

 as in the living cell already described. §§ In the semi- 

 liquid protoplasm are embedded numerous green oval 

 bodies called " chloroplasts." These are distinct masses 

 of protoplasm which contain the chlorophyll, and to 

 them is due the green colour of the leaf. It is withia 

 the chloroplasts that the process of assimilation goes 

 on ; if they are observed under the microscope while 

 the leaf is assimilating, minute granules of starch may 



Xt F. F. Blackman, 1' hilosophical Traiisacliotu of the Soyal 

 SoeUty, 1895. 



§§ KxcwiEDQE, January, 1900. 



be seen to appear in uu^ni. i.ivi, >\v aic able to identify 

 the exact spots where this wonderful process of the 

 conversion of inorganic Carbon into organic is carried 

 on. It is upon the work performed in these minute 

 chloroplasts that the whole organic world — plants and 

 animals — depends for its supply of Carbon ; for animais 

 as well as those plants and parts of plants wliich con- 

 tain no chlorophyll can only obtain their Carbon from 

 organic substances. 



Fio. 2.— A portion of a Trans- Fio. 3.— -V small part of 

 verse Section through a Leaf of tlie Fig. 2, enlarged, s, a Stoma. 

 Cherry Laurel, s, a Stoma. I c, G, Guard-Cells, x 400. 

 spaces between the cells into wliich 

 stomata open. x 100. 



What is the secret of this remarkable power which 

 chlorophyll possesses? To this question no satisfactory 

 answer can at present be given. Before the processes 

 which are carried on in the chloroplasts can be under- 

 stood we must learn much more about protophism thaa 

 is yet known. We have seen that chlorophyll cannot 

 decompose Cai'bon dioxide and build up organic sub- 

 stances in the dark. If we try to decompose Carbon 

 dioxide by artificial means we find that it is difficult, 

 and can only be effected by a very high temperature. 

 The heat wliich we apply is transformed into another 

 form of energy, which forces apart the atoms of Carbon 

 and Oxygen and so bi-iugs about the decomposition. 

 The separation of these elements when tliey are com- 

 bined in the form of Carbon dioxide is only possible 

 when there is a supply of energy from outside. When 

 it takes place in the chloroplasts of the leaf, this energy 

 is undoubtedly derived from light. Ordinary sunlight, 

 however, is incapable of effecting the change by itself. 

 In passing through a solution of chlorophyll, whita 

 light undergoes alteration, some of its constituents being 

 absorbed by the chlorophyll and others passing 

 through. The secret of the decomposition of Carbon 

 dioxide in the chloroplasts lies in this fact, viz., that 

 some of the constituent colcurs of sunlight are arrested 

 and others transmitted by the chlorophyll. In ths 

 present state of knowledge it is impossible to go farther 

 than this without becoming involved in speculations 

 which are insufficiently supported by practical experi- 

 ment. We must for the present rest content with the 

 statement that the assimilation of Carbon dioxide goes 

 on only in protoplasm which contains chlorophyll, and 

 under the influence of light. 



All green plants, under favourable conditions, assimi- 

 late Carbon dioxide, and undoubtedly obtain the greater 

 part of their Carbon in this way; it is, however, by no 

 means certain that some of it docs not come from organic 

 substances in the soil. We shall have another occasion 



