8 BOTANY OF THE LIVING PLANT 



in the soil show that the behaviour of these two parts of the seedling 

 is constant, and suggest that it is a response to some external influence 

 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 VIII.). 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 response which the living 

 organism shows is called the reaction. 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 reaction to stimulus, and such 

 a reaction 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 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 VIII. 



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. 



