THE STEM. 17 



ish without it, we find that the root is essential to the life 

 and well-being of the plant. Consequently we see that 

 the root is an organ. 



CHAPTER III. 

 The Stem, 



. Sectio2^ I. — KiXDS OF Stem. 



31. The stem is that part of the plant which supports 

 the leaves, flowers, and fruit. The three great classes of 

 Exogens, Endogens, and Acrogens constitute the first 

 grand divisions of stems. These names express their dif- 

 ferent manner of growth, and mean simply outside growth, 

 inside growth, and increasing in growth from the ex- 

 tremity. 



32. Exogenous plants grow by successive additions to 

 the outside of the wood, a ring being formed each year, 

 between the previous year's growth and the bark. They 

 are furthermore distinguished by a light elastic substance 

 occupying the centre of the stem, and well known under 

 the familiar name of pith. 



33. Medulla is its botanical name. This is large in 

 young plants, but becomes partly absorbed by the growth 

 of the plant, and partly compressed into thin plates that 

 are found to extend through the woody portions, terminat- 

 ing in the bark, and dividing the w^ood into wedge-shaped 

 portions. These divisions are called medullary rays. They 



31. What is the stem? What is the first division of stems? What do their 

 names imply ? 



32. Huw do cxogeuous plants grow? What furthermore distinguishes them ? 



33. Is this medulla larger in young plants? What becomes of it? What are 

 the transverse lines of pith called ? 



