SEEDS AND SEEDLINGS. 41 



The young plant in a "Brazil nut" is very rich in oil, as can be seen 

 on cutting the "nut" across, after removing the shell : the oil is made 

 very evident if the knife be heated. 



70. Digestion of Reserve Pood. The foods stored up 

 in seeds for the use of the young plants must be insoluble in 

 water, and not able to diffuse through membranes (cell- 

 walls) : why ? The reserve food in the Bean cotyledons 

 must, however, when the seed germinates, travel from cell to 

 cell through the cell- walls in order to reach the places where 

 it is needed (radicle and plumule) , and in order to do this the 

 food must be changed into soluble and diffusible substances. 

 The food stored in a seed must, in fact, be digested before 

 it can be used by the young root and shoot, and the process 

 of digestion in a plant is essentially the same as in an animal. 

 Find out all you can, from books on Physiology, about the 

 digestion of starch, proteids, and fats. In the germinating 

 seed, just as in an animal, starch is converted into sugar, 

 proteids into peptones, oils into fatty acids and glycerine 

 (which are further changed into sugars, etc.). 



The conversion of the insoluble reserve foods into soluble 

 foods is brought about by the action of ferments, correspond- 

 ing to those secreted by the digestive glands of an animal's 

 alimentary canal. Starch is changed into sugar by the fer- 

 ment diastase, similar to the " ptyalin " of saliva and the 

 " amylopsin " of pancreatic juice ; proteids into peptones by 

 trypsin (also contained in the pancreatic juice of animals) ; 

 oils into glycerine and fatty acids by lipase (corresponding to 

 the " steapsin " of pancreatic juice). 



71. How long does the Seed's Pood last? We shall 

 see later that green plants get their food from two main 

 sources, air and soil. The young plant in a seed has a store 

 of food for its early growth, a store which is sometimes very 

 scanty and sometimes (as in Bean and Pea) very abundant. 

 We know that tap- water and even rain-water are not pure, 

 but contain dissolved substances, while soil-water or river- 

 water will be much richer in dissolved matter. In order to 

 find out how long the stored food lasts, we should therefore 

 use distilled water, so that we know exactly what the roots 

 have had at their disposal; we may use well- washed sand 

 and water it with distilled water. 



