Growth of Roots ^ Stems, and Leaves 23 



Ex. g. The cotyledons of the bean have no petioles. On the stem of 

 those which are turned away from the light, below the cotyledons, make 

 a row of ink marks. Place in a lighted window. In a day or so the 

 cotyledons will look toward the light. The row of marks will show that 

 the stem has twisted to bring them into position; When growth in that 

 portion of the stem has stopped, turn the plant halfway around. Will the 

 cotyledons again turn toward the light ? 



Ex. 10. Remove the tips from sunflower seedlings, cutting off some 

 above and some below the cotyledons. Do the stems still curve toward 

 the light ? 



Ex. II. Plant sunflower seeds and keep them covered so as to exclude 

 all light. At the same time plant others and leave them exposed to light. 

 When the cotyledons of the second lot are well expanded, compare with 

 those which have been covered. The cotyledons are still closely pressed 

 together. Cover those that were left exposed. After a day or two ex- 

 amine them again. Evidently light has an influence in spreading leaves 

 apart ; in darkness they close. 



Notice Oxalis plants at night. Leaves and flowers are all 

 closed. In the morning they open. On very 

 bright days the leaves go to sleep, while the 

 flowers remain open. On cold days they remain 

 closed. There is a cushion-like joint at the base 

 of the leaflets made of thin-walled cells. When 

 the cells on the underside lose a portion of their 

 contents the joint bends and the leaf closes down. 

 Look for joints in clover. Are they in the same 

 position ? Do clover leaves lie down or stand up 

 when they go to sleep ? 



If a plant remains in darkness, the leaves re- 

 main small and undeveloped, while the internodes 

 lengthen. The plant in Fig. 30 grew under the ^iq 01 —xhe 

 shelter of a large rock. The part in the pot grew baxed-up Kar- 

 on the sunny northern side, while the piece at the ^^^^i T'shoot 

 left grew in a more shaded place on the southern i" search of 



• J T- • .1 • . r . light. Observe 



Side. l^ig. 31 IS the picture of a very compact the difference 



little plant when it is grovving at home near Beau- in position and 



r .17 1 , -1 1 • -11 • , • , . s'2^ of leaves. 



tort West ; but while lying in the box m which it 



was posted, the tip of the plant started on a journey of its 

 own. During this growth the leaves formed were very small, 

 but the lengthening stem placed them far apart. It has long 

 been known that a plant grows faster at night and it is a 



