HOW PLANTS GROW. 597 



the plant, the plant makes its woody growth in every direction. 

 The active principle or substance which has life in a tree 

 is called by botanists the protoplasm. This substance goes 

 through all the young growing cells, and carries with it all the 

 substances which the leaves have made. It takes them to every 

 point where new material is needed, and may be called the hod- 

 carrier of the plant. Even the roots owe their growth to the 

 substances which the leaves have made, and which this active 

 agent of life brings to them. The new shoots get their part at 

 the growing tips, and new leaves are made to carry on the work. 

 All down between the bark and the wood of our trees, this 

 agent of life passes and makes new rows of cells, to the young 

 wood on one side, and to the young bark on the other. And 

 then, after these rows of cells are made, year after year, around 

 a tree trunk, it fills them up and adds other materials, until they 

 are finished and made into heart wood, as we call it. Heart 

 wood, then, is finished wood from which the life principle has 

 gone, and which is really dead wood. Life in a tree exists only 

 in the sap-wood, and more actively, in our ordinary trees, in the 

 young part between the wood and bark. All parts of a plant, 

 then, roots, stems, branches, and leaves, are made in the great 

 laboratory carried on by the leaves and green parts of the plants, 

 and the material formed by the leaves is transported up, down, 

 or in any direction, when new growth is going on. 



This shows how important it is that we have a full develop- 

 ment of foliage and green tops on our plants, in order that the 

 work may be well done. This also shows that a full foliage 

 indicates that the plant or tree is in health, receiving and assimi- 

 lating food in such quantities as are either promoting growth 

 or preserving life unimpared. 



What Food Plants get from the Soil. We have seen that 

 the chief thing, if not the only one, that plants get from the air, 

 and with which they make the greater part of their bulk, is an 

 element called carbon, which they get by decomposing the com- 

 pound called carbon dioxide. Let us now see what they got 

 from the soil. These things we have found are left in the ashes, 

 when we found the wood or organic matter, and returned the 

 carbon dioxide to the air. The substances we find in the ashes 

 of a plant are called the ash elements. Now an element is a 



