CHAPTER 1. GEN REAL PHYSIOLOGY. 685 



The physiological adaptation of stems is such that the move- 

 ments which they perform in response to the action of external 

 stimuli are always such as shall place the foliage-leaves and the 

 reproductive organs in the most favourable position. Some stems, 

 for instance, grow away from the centre of the earth (negative 

 geotropism) and towards the light (positive heliotropism) ; these 

 stems consequently grow up into the air, and take up such a 

 position with regard to the direction of the incident rays of light 

 that the leaves may be adequately exposed to them. Others, 

 again, grow horizontally under the influence of gravity (diageo- 

 tropism) and of light (diaheliotropism), and in this way spread 

 out their leaves to the sun's rays. 



In some cases stems which tend to grow erect into the air are 

 unable to do so in consequence of being insufficiently rigid to 

 maintain their own weight, and that of their leaves, etc. Such 

 stems are enabled to obtain the necessary support by becoming 

 attached to foreign bodies, such as other plants, rocks, etc. This 

 attachment is sometimes purely accidental, as in the case of the 

 hook-climbers, such as the Bramble, where the stem is covered with 

 prickles which become fixed as the swaying shoot is blown about 

 by the wind. Bat in other cases the attachment is the result of 

 the mode of growth of the stem or its branches, in virtue of which 

 they twine round any suitable foreign body with which they may 

 come in contact. In some cases the stem and its branches are 

 sensitive to contact, e.g. Dodder ; in others, this sensitiveness is 

 restricted to certain specially modified branches, termed tendrils 

 (see p. 43, e.g. Vitis, Passiflora), and it is possessed by them in a 

 very high degree. 



c. THE LEAF. In the discussion of the morphology of the leaf it 

 was pointed out that the forms of leaves are very various ; so 

 much so that it was necessary to classify them into a number of 

 categories. Each of these will now be briefly considered with 

 regard to its functions. 



(1). Foliage-leaves. It may be stated generally with reference 

 to land-plants, that the two great functions subserved by the leaf 

 are, first, the construction of organic substance from the raw 

 materials of the food; and second, the exhalation of watery vapour, 

 or transpiration. 



The internal structure of the leaf is in direct relation to these 

 two functions (see p. 162). The particular significance of the 

 form and arrangement of the cells of the mesophyll is made clear 



