INTRODUCTION 3 



parent and into a xylem element if it be born on the other ? 

 Is it due to some subtle influence or stimulus which has its 

 origin in the adjacent structural elements ; or is it due to 

 some quality in the cell itself, an heredital predetermination. 



To these questions there are no real answers ; the facts 

 must be accepted, their explanation must be left to the 

 future. 



The embryo grows and develops into the autotrophic 

 organism of a form and structure determined by its conditions 

 of life and by its ancestry and exhibiting those actions and 

 reactions commonly associated with the higher plants. The 

 shoots and roots circumnutate and respond to various stimuli, 

 gravity and light being the most obvious. Circumnutative 

 and other autonomous movements may be explained by such 

 conceptions as rectipetality and associated engrams ; whilst 

 in explanation of tropisms various mechanistic hypotheses 

 have been formulated ; some chemical, the hormone theory 

 of gravitational stimulus of roots, for instance ; others physical, 

 the statolith theory, for example. 



The highly organized root system by means of its root 

 hairs takes up raw material by osmosis in the form of water 

 and its dissolved salts ; in many plants, possibly in all, the 

 osmotic strength of the cell sap of the root hairs is continu- 

 ously adapted and is nicely adjusted to the osmotic strength 

 of the soil water. From the root hairs water is passed on 

 through the cortex to the water-conducting elements of the 

 vascular cylinder, and thus supplies the shoot system. The 

 shoot system, no less highly organized, is, in the first instance, 

 concerned with the manufacture of food, carbohydrate, fat and 

 protein. In this connection the leaf, a marvel of organization 

 — with its chlorophyll apparatus supported by the network of 

 veins which also are the conduits for the conveyance of fluid 

 raw materials and for the elaborated products, and with its 

 mechanism for the regulation of gaseous interchange — is the 

 great synthetic factory, building up food apparently with the 

 greatest ease and certainly with remarkable rapidity. 



In due season reproduction takes place. Of the problems 

 here involved the secretion of nectar, when it obtains ; the 



