STRUCTURE AND FUNCTIONS 17 



entire stem is enclosed in an epidermis. Each fibro- 

 vascular bundle is made up of three parts, the outer 

 part or phloem through which the organic or elaborated 

 food substances are carried, the inner woody part or 

 xylem through which the crude substances (water and 

 salts) pass, and a delicate or growing part between the 

 two which is known as the cambium. In the very 

 hard and woody plants these fibro-vascular bundles are 

 packed closely together and there is a minimum amount 

 of cortex and pith. In the mono- 

 cotyledonous (Fig. 10) plants, such 

 as corn, palm, etc., these fibro- 

 vascular bundles are scattered irre- 

 gularly through a mass of pith. 

 The fibro - vascular bundles are 

 extended into the leaves, where 

 they divide and subdivide re- 

 peatedly, thus forming the intri- 

 cate venation which supports the IG ' 

 softer parts of the leaf. Therefore 

 all parts of the plant are brought into the most intimate 

 communication. 



The parts of the flower are modified leaves which 

 are set aside for specific duties. The structure of the 

 lower groups is less complicated than that of the 

 Spermatophytes, the complexity decreasing as we pass 

 from the Spermatophytes to the algae. 



PHYSIOLOGY 



Although we have a fair understanding of the 

 more conspicuous functions of the green plants, much 

 more remains to be learned. The leaves are the real 

 laboratories of the plant. They take in the carbon 

 dioxide (C0 2 ) from the air, and receive from the roots, 

 by means of the fibro-vascular bundles, the water (H 2 0) 

 which has been taken in by the root hairs. Then by 

 means of the sunlight acting upon the chlorophyll they 

 tear down and reconstruct these two substances into 



c 



