SUGGESTIONS.—_Forms of stems._ =43.= Are the trunks of trees ever perfectly cylindrical? If not, what may cause the irregularities? Do trunks often grow more on one side than the other? =44.= Slit a rapidly growing limb, in spring, with a knife blade, and watch the result during the season. =45.= Examine the woodpile, and observe the variations in thickness of the annual rings, and especially of the same ring at different places in the circumference. Cross-sections of horizontal branches are interesting in this connection. =46.= Note the enlargement at the base of a branch, and determine whether this enlargement or bulge is larger on long, horizontal limbs than on upright ones. Why does this bulge develop? Does it serve as a brace to the limb, and is it developed as the result of constant strain? =47.= _Strength of stems._ The pupil should observe the fact that a stem has wonderful strength. Compare the proportionate height, diameter, and weight of a grass stem with those of the slenderest tower or steeple. Which has the greater strength? Which the greater height? Which will withstand the most wind? Note that the grass stem will regain its position even if its top is bent to the ground. Note how plants are weighted down after a heavy rain and how they recover themselves. =48.= Split a cornstalk and observe how the joints are tied together and braced with fibres. Are there similar fibres in stems of pigweed, cotton, sunflower, hollyhock? [Illustration: FIG. 68.—POTATO. What are roots, and what stems? Has the plant more than one kind of stem? more than two kinds? Explain.] CHAPTER X THE STEM—ITS GENERAL STRUCTURE There are two main types of stem structure in flowering plants, the differences being based on the arrangement of bundles or strands of tissue. These types are _endogenous_ and _exogenous_ (page 20). It will require patient laboratory work to understand what these types and structures are. =Endogenous, or Monocotyledonous Stems.=—Examples of endogenous stems are all the grasses, cane-brake, sugar-cane, smilax or green-brier, palms, banana, canna, bamboo, lilies, yucca, asparagus, all the cereal grains. For our study, a cornstalk may be used as a type. [Illustration: FIG. 69.—CROSS-SECTION OF CORNSTALK, showing the scattered fibro-vascular bundles. Slightly enlarged.]