Janiary 1, 1900.] 



KNOWLEDGE. 



the substance of the plant. Strange r.s this idea 

 appeai-s to us, it did not finally die out until the 

 XVIIIth century. Passing over the writings of several 

 philosophers who were influenced in a greater or less 

 degree by these ideas of Aristotle, we will notice the 

 views of Van Helniont,* a Dutch jihysician of the 



Fig. 1. (After Haustein.)- — A living plant cell, very highly magni- 

 6ed. A. — The cellulose wall. B. — The wall of a neighboiirine cell, 

 which is not represented, c. — Inner wall of protoplasm (Primor- 

 dial L'trii-le). D. — Strands of protoplasm connecting the Primordial 

 Ctricle with the central mass of protoplasm (E),in which the nucleus 

 (n) is embedded. — From the figure in Vine's " Physiology of Plants." 



XVTth century. He believed water to be the princi- 

 pal constituent of matter, and therefore of the body of 

 the plant. Hence he considered that the food of 

 plants consisted solely of pure water. This idea is 

 nearly as far from the truth as that of Aristotle, but 

 it is liistorically of great importance because it was 

 supported by au experiment — as far as is known, the 

 first botanical experiment ever performed. He placed 

 in a pot 200 lbs. of dried earth, and in it he planted a 

 willow branch which weighed 5 lbs. He kept the 

 whole covered up and daily watered the earth with 

 rain-water. After 5 years' growth the willow was 

 taken up and again weighed and was found to have 

 gained 164 lbs. ; the earth in the pot was dried and 

 weighed and had only lost 2 oz. Knowledge was not yet 

 sufficiently advanced to enable Van Helmont to inter- 

 pret these striking results correctly, and he came to 

 the erroneous conclusion that the increased weight of 

 the plant was due to the water which had been supplied 

 to the roots. He therefore looked upon this experi- 

 ment as supporting the theory which he had advanced, 

 viz., that plants required no food but water. But 

 although his conclusions were wrong, yet to him is due 

 the honour of having been the first to adopt the experi- 

 mental method of enquiry in investigating problems 

 connected with plant life. A hundred years later a 



• Bom in Urussels, 1.577. 



very important advance was made by Stephen Hales. f 

 This distinguished English physicist was the first to 

 prove that part at least of the food materials of plants 

 is derived from the atmosphere. The vast import- 

 ance of this discovery will be realised when we consider 

 the assimilation of the Carbon dioxide (Carbonic acid 

 gas) of the atmosphere by the leaves of green plants. 

 Soon after the death of Hales it was shown that the 

 Carbon of the plant is derived from Carbon dioxide of 

 the air, and that at the same time the water of the soil 

 containing nitrates and other mineral matters in 

 solution is taken in by the roots and utilised as food 

 material. The true interpretation of Van Helmont's 

 experiment was that the increase in weight of the 

 willow branch during the 5 years' growth was in a 

 great measure duo to Carbon which it had taken from 

 the air. 



By these discoveries a firm foundation for further in- 

 vestigation was laid. Although the progress made since 

 the time of Hales has been far from uniform, it has 

 been great, especially during the last tO years, as we 

 shall see in our further consideration of the subject. 

 There is nevertheless a wide scope for furtlier en((uiry, 

 and much remains yet to be discovered before all the 

 problems connected with plants and their food can be 

 satisfactorily solved. 



A leaf-bearing plant iii;iy bo looked upon as a re- 

 public whose units are called " ccll,s.'' A large pro- 

 portion of the cells of a living plant arc dead, although 

 they have by no means ceased to be of u.se to the plant ; 

 such are the hard fibres of which the wood of trees is 

 largely composed, and which give to the trunk the 

 rigidity which enables it to sustain the heavy weight 

 of the branches and foliage. It is however with the 

 living cells that we are chiefly concerned, in consider- 

 ing plants in relation to their food. We may regard a 

 living cell as a very minute bag or sac bounded by a 

 double wall. The size of the cell varies in difi'erent 

 plants and in different parts of the same plant. Some 

 idea of the average size of such ceils may be conveyed 

 by the statement that between 6000 and "l 2,000, spread 

 out in a single layer would cover 1 square inch of sur- 

 face. The outer wall of the cell (the cell-wall proper) 

 is composed of an elastic]: substance called " cellulose,' 

 which in its chemical properties resembles starch. A 

 cellulose wall permits the passage of liquids through it, 

 and must therefore be minutely perforated, though the 

 perforations are so small as to be quite invisible under 

 the highest powers of the microscope. The inner wall, 

 sometimes called the '' Primordial Utricle," closely lines 

 the outer and is composed of a viscid substance called 

 protoplasm, strands of which pass from points in the 

 wall to the interior of the cell, and there unite form- 

 ing a mass in which is embedded an oval body, the 

 nucleus. The nucleus is also composed of protoplasm, 

 but it possesses many remarkable properties which 

 mark it out as a distinct body. Protoplasm consists of 

 a mixture of substances called proteids, which are com- 

 posed of Carbon, Hydrogen, Nitrogen, Oxygen, and one 

 or two other elements. Its most remarkable character is 

 that it possesses properties which lead us to speak of it 

 as a living substance. Some of these properties will 

 come under our notice at a later stage. As much of 

 the cell as is not occupied by protoplasm is filled by a 



t Born in Kent, 1677. Entered Christ's College, Cambridge, 169«. 

 Fellow of the Royal Society. 



X An " elastic " substance possesses the property of returning to 

 its original shape after being stretched. 



