IRRITABILITY. 



variation. Such leaves tend to arrange themselves in a vertical or para- 

 heliotropic position, in which the surfaces are not exposed to the incidence 

 of light of the greatest intensity, but to the incidence of the rays of diffused 

 light. Interesting cases of the fixed position of leaves are found in the so- 

 called compass plants (like Silphium laciniatum, Lactuca scariola, etc.). In 

 these the horizontal leaves arrange themselves with the surfaces vertical, and 

 also pointing north and south, so that the surfaces face east and west. 



268. Importance of these movements. Not only are the leaves placed in 

 a position favorable for the absorption of the rays of light which are con- 

 cerned in making carbon available for food, but they derive other forms of 

 energy from the light, as heat, which is absorbed during the day. Then 

 with the nocturnal position, the leaves being drooped down toward the stem, 

 or with the margin toward the sky, or with the cotyledons as in the pump- 

 kin, castor-oil bean, etc., clasped upward together, the loss of heat by 

 radiation is less than it would be if the upper surfaces of the leaves were 

 exposed to the sky. 



269. Influence of light on the structure of the leaf. In our study of the 

 structure of a leaf we found that in the ivy leaf the palisade cells were on 



the upper surface. This is the case with a 

 great many leaves, and is the normal arrange- 

 ment of " dorsiventral " leaves which are dia- 

 heliotropic. Leaves which are paraheliotropic 

 tend to have palisade cells on both surfaces. 

 The palisade layer of cells as we have seen is 

 made up of cells lying very close together, and 

 they thus prevent rapid evaporation. They 

 also check to some extent the. entrance of the 

 rays of light, at least more so than the loose 

 spongy parenchyma cells do. Leaves developed 

 in the shade have looser palisade and paren- 

 chyma cells. In the case of some plants, if 

 we turn over a very young leaf, so that the 

 under side will be uppermost, this side will 

 develop the palisade layer. This shows that 

 light has a great influence on the structure of 

 the leaf. 



270. Movement influenced by contact. In 

 the case of tendrils, twining leaves, or stems, 

 the irritability to contact is shown in a move- 

 ment of the tendril, etc., toward the object in 

 touch. This causes the tendril or stem to coil 

 around the object for support. The stimulus is also extended down the part 

 of the tendril below the point of contact (see fig. 123), and that part coils 



Fig. 123. 

 Coiling tendril of bryony. 



