8 BOTANY OF THE LIVING PLANT 



that applies to them all. In this case the influence is Gravity. The 



subject of external influences will be discussed in detail later (see 



Chapter IX.). Meanwhile it must suffice to say that an influence 



such as Gravity, which acts on a living organism so as to produce 



a change in it, is called a stimulus. The effect which the living 



organism shows is called the response. The effect of Gravity upon 



the growing shoot or root, so as to make the one turn upwards and 



the other downwards, is an example of response to stimulus, and such 



a response is one of the essential indications of Life. 



It is familiar to every gardener that, up to a certain point, the 



higher the temperature the quicker his seedlings appear above 



ground. But plants vary in their relation to temperature, and that 



necessary for germination is not the same for them all. Thus most 



cereals can germinate at a temperature very near to the freezing 



point, whereas Maize and the Kidney Bean require a minimum 



temperature of about 9° C. All the functional activities of the Living 



Plant have such a relation to temperature. The case of germination is 



merely one example of a general condition of Life. This subject will 



also be taken up again in Chapter IX. 

 The root and shoot established on germination are capable of 



continued growth, which is followed in both cases by the formation 



of lateral appendages. Thus a Root-System and a Shoot-System 



are established, the former being buried in the soil, the latter rising 



above the level of the soil, and constituting the part of the plant 



ordinarily seen (Fig. 2, v.). If the soil be carefully washed away from 



the root-system of a Bean-seedling after the main root has attained 



about eight inches in length, it will be seen to consist of a primary, or 



tap-root, which grows directly downwards and bears horizontal lateral 



roots. The smallest and youngest of these are nearest to the tip of the 



main root, and the largest and oldest are most remote from it. These 



may in like manner bear lateral roots of still higher order, radiating 



in all directions. Thus a complex root-system is built up. The 



extreme tip of each root comes naked out of the soil, and is pellucid 



and slimy to the touch, so that it readily slides past obstacles as it 



penetrates the soil. But about three-quarters of an inch back from 



the tip the particles of soil adhere to the root, showing that from that 



point backwards a close relation is established between the root and 



the soil. It will be seen later that this is due to the presence of 



numerous minute root-hairs. 



When the shoot of the Bean has grown to the length of about 



six inches, it will be seen to consist of a central Stem terminated by 



