356 



BOTANY: PRINCIPLES AND PROBLEMS 



In perennial herbs the underground parts survive and it is only 

 the upper portions which die back; in biennial herbs, the plant 

 lives through two seasons, storing up food the first and flowering 

 the second, and in annual herbs the plant body lives through 

 only one season, and survives unfavorable conditions in the seed. 

 Herbs are now very rich in species and include the majority of 

 our food plants and many others of economic importance. 



4nthGr 

 -Filament 



Sepal 



Petal Stamen 



(f 



Pisti 



-S+igma 

 -Style 



- Ovary 



-Chambers 

 of Ovary 



■ Ovule 



Cross Scciior, 

 of Ovary 



Fig. 227. — The structure of the flower of a dicotyledonous seed-plant (dia- 

 grammatic). A, face view of the flower, showing its calyx of five sepals, its 

 corolla of five petals, its ten stamens and its pistil. B, longitudinal section, 

 showing the relations between the parts. 1, Receptacle. 2, Calyx. 3, Corolla. 

 4, Stamen. 5, Pistil, with ovary cut lengthwise. 



Internally, the vascular system reaches in angiosperms its 

 highest degree of specialization. A cambium is well developed 

 in the woody members but is much reduced among herbs. The 

 wood consists not only of those general-utility elements, the 

 tracheids, but also of thick-walled fibers whose function is to 

 furnish rigidity to the stem; and of wide, thin-walled ducts or 

 vessels, by which large quantities of water can be conveyed rapidly 

 through the wood. In the bast, too, a new element, the com- 

 panion-cell, intimately related to the sieve-tube, makes its 

 appearance. 



