18 FAMILIAR LESSONS IN BOTANY: 



are very plain to be seen in a transverse section of any 

 exogenous stem, particularly some varieties of the oak. 

 It will generally be found that the larger the space occu- 

 pied by the medulla or pitli, the harder is the surrounding 

 wood, a wise provision of Providence to compensate for the 

 weakness of the interior. For instance, the cane (Arimcli- 

 naria macrosjyerina), three-fiftlis of whose diameter is occu- 

 pied by pith, has its surrounding wood almost as hard as a 

 bone. 



34. The ExDOGEXOus plants increase by internal growth 

 and elongation at the summit, the wood growing in bun- 

 dles and irregular distribution through the whole mass; 

 the outside presenting more the appearance of scales and 

 scars from the falling off of the leaves, than the bark proper, 

 as seen in the exogenous family. 



35. A botanist needs only a bit of stem of each of these 

 to determine immediately the family to which it belongs. 

 The first, exogen, will present a series of rings ; in the 

 centre will be the pith — surrounding this, concentric layers 

 of wood, and then the bark ; while the latter, endogen, has 

 no pith, and the wood consists of fibrous bundles distrib- 

 uted in irregular masses through the cellular tissue of the 

 stem, and with no true bark. You may be surprised to 

 learn that from these same bits of stem he can tell you 

 how the embryo, the first rudiments of the plant of each, 

 appeared ; the exogenous embryo having two seed-leaves 

 (only a few have several in a whorl), with their true leaves 

 net-veined, as in Fig. 11 ; while the embryo of the eudogen 

 plant has one cotyledon, and its true leaves are parallel- 

 veined, as in Fig. 12. 



36. ACROGEXOUS means increasing in growth from the 

 extremity. They are plants which grow taller, but do not 



34. How does the outside appear ? 



35. What does a botanist only need, to determine one of these families ? How 

 will the exogenous stem appear? The endogenous? What further can he tell? 

 What kind of leaf has the exogen ? The endogen ? 



36. What plants arc found in the exogcn? Endogen? Acrogen? 



