41] CHAPTER I. GENERAL, PHYSIOLOGY. 171 



of which the water or the reserve-materials are deposited. Or 

 they may be developed into thorns (p. 27) as a protection against 

 being eaten. 



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. Most 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 posi- 

 tion 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 (diageotropism) 

 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 main- 

 tain 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 attach- 

 ment 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. But 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. 27, 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. 



