140 PHYSIOLOGICAL BOTANY. 



Vessels of cienchyma pervade the under-layer of parenchyma, 

 returning the elaborated juices through the petiole into the 

 cambium layer. 



Review. 421. Note a cross-section of Endogen. Its structure. 422. Constituents of a 

 wood-bundle (explain 511). 423. Trac its course. Has the Endogen a true bark ? Why 

 so difficult to split? 424. Show the significance of the term Endogen. Also of the term 

 Exogen. Why the caudex is smallest at base. 425. What tribes are Acrogens ? Signifi- 

 cance of the term. Carefully describe a cross-section (514). 42H. What is the rank of the 

 Thallogens? Significance of the term? Their only tissue? What semblance of stems 

 have they? 428. Fibrilke of the Eoot. Show them. 429. Pileorhiza. Show it. 430. How 

 the root g^ows and penetrates the soil. 431. Origin of the leaf. Substance of the veins. 

 432. The parenchyma. When are its two strata alike ? 434. Place of the stomata? Ex- 

 ceptions. 435. Condition of the chlorophyl. The cienchyma, where and why ? 



CHAPTER VI. 



VEGETATION, OK THE PHYSIOLOGY OF PLANT LIFE. 



436. We have now briefly surveyed the mechanism of the 

 plant both its outward forms and internal structure. We next 

 inquire into the uses of all this wonderful apparatus ; what the 

 specific office which each part performs in the economy of the 

 plant ; arid how all parts co-operate in the work of living and 

 growing. 



437. What is life? This inquiry meets us at the beginning 

 a problem never solved. The spontaneous action of the plant, 

 the self-determined shapes which it assumes, we at once refer to 

 this principle its Vitality ; but of the nature of this principle 

 itself, we can only say Is it not a direct emanation from the 

 Supreme Will, the Fountain of all life ? 



438. Vegetation is doubtless the lowest form of li-fe. It 

 springs directly from inorganic or mineral matter, and is the 

 first step in the organization of mineral matter. Its material is, 

 therefore, mineral matter rendered organic through the vital 

 force. The subordination of the vegetable to the animal king 

 dom is thus manifest in its being fed and nourished on inorganic 

 matter. It is interposed between these two incompatible ex- 

 tremes, and is ordained to transform the innutritions mineral 

 into the proper and indispensable food of the animal kingdom. 



439. The process of vegetation consists of imbibing the crude 

 matters of the earth and air, transforming into sap, assimilating 



