RELATION OF STRUCTURE TO FUNCTION IN ROOTS. 749 



ornamental form of branching is produced which may even be recognized on the 

 older boughs, and gives a characteristic habit to the shrub or tree. Although the 

 annual growth in height in woody plants branching in this fashion is only slight, 

 the crown grows in breadth to a striking degree, and the older, leafless boughs 

 have usually the appearance of horns or of an interwoven lattice- work spreading 

 out above, as may be seen in a remarkable manner in the Stag's-horn Sumach 

 (Rhiis typhina), and also in several species of Horse Chestnut (e.g. JEsculus Jiava 

 and discolor). In the Oleander {Nerium Oleander), and frequently in the 

 Mistletoe (Viscum album, c.f. fig. 46, p. 206), the scarred apex of the main shoot 

 is overtopped by a whorl of three lateral shoots, which produces another charac- 

 teristic modification of this form of branching. 



The internal structure of the floral stem, especially the arrangement of the 

 mechanical tissue, is always adapted to the tasks naturally assigned to the bearer 

 of the flowers and fruits. When the floral portions and the fruits proceeding from 

 them are to be maintained in an erect position, the stalks and also the rachis in 

 question are constructed so as to resist flexion. The stalk and rachis of pendent 

 flowers, and especially of heavy pendent fruits, are, on the other hand, constructed 

 to resist tension; in both cases they are provided with mechanical tissue 

 suitably strengthened and arranged. Frequently the same bast cylinder which 

 afibrded the resistance to flexion in the erect flower-stalk at the time of the 

 opening of the flowers has subsequently to provide a resistance to strain, as when a 

 pendent fruit is produced from an erect flower. The converse also happens, and not 

 infrequently erect fruit-stalks, very resistant to bending, which take part in the 

 dispersion of the seeds, are developed from pendent flower-stalks with the capacity 

 of resisting strain. For the rest, in all these alterations of position, the turgescence 

 of the parenchymatous tissue on the periphery of the flower-stalk plays a prominent 

 part. 



4. FORMS OF ROOTS. 



Relation of external and internal structure to function. — Definition of the root. 

 — Remarkable properties of roots. 



RELATION OF EXTERNAL AND INTERNAL STRUCTURE TO FUNCTION. 



Every seed is provided by the parent plant with as much starch, fat, sugar, 

 and other materials as are necessary for its further independent development. 

 The germinating seed respires, provides itself with water, dissolves the materials 

 stored up in its cells, augments the number of its cells, and increases in size. The 

 food-substances of the soil at first take little or no part in these processes. But 

 as the seed germinates its aim is to develop organs capable of laying the food- 

 substances contained in the soil and air under contribution, and of manufacturing 



