AWAKENING OF BUDS AND THE SLEEPING OF LEAVES. 



303 



radiation of heat, and it is not until they are pretty nearly full-grown 

 that they finally acquire a horizontal position. 



Fig. 161 illustrates the former state of the leaves, and if this be com- 

 pared with the accompanying figure of a Lupine when " asleep " at night 

 (fig. 162), it will be seen that the position of the leaflets is the same, for 

 the object is identical. The Lupine, moreover, is an adult leaf, and sleeps 

 periodically, the leaflets falling at eve and rising at dawn. 



The Walnut has a pinnate leaf like the Ash, with four or five pairs of 

 leaflets. As soon as the leaf can escape from the bud the main petiole 

 curves strongly downwards (fig. 163). The leaflets, as is usual with most 

 plants, as Horse-chestnut, Kose, Laburnum (fig. 168), &c., are " condupli- 

 cate," i.e. the two halves of the blade are folded tightly together, like a 

 sheet of note-paper. This protects the upper surfaces, while an additional 

 advantage is gained by the blades being placed at first vertically. As the 

 pairs of leaflets are developed they open out and become horizontal, one 

 pair after another, until the whole leaf is mature. 



In the Ash the dark brown buds consist of petiolar scales, and if 

 examined in spring they aflbrd an excellent illustration of transitions 



Fig. 161. Fig. 162. Fig. 163. 



Horse-chestnut. Lupinus pilosus. Walnut. 



Leaf, seen laterally, asleep 

 at night-time. After Darwin. 



between petiolar scales and true leaves. Instead of curving downwards 

 on expansion, as in the Walnut, the petioles are erect, but this still enables 

 the undeveloped leaves to stand in a vertical plane. 



In the winter, however, on dissecting a bud after removing the scales, 

 a pellet of dark brown wool is seen, and on teazing this out, little white 

 leaves will be found on which the wool grows. The hairs are filled with 

 pale brown liquid and granules. 



Stipular Scales. — Many of the plants are provided with a pair of small 

 appendages at the base of the leaf. These are called "stipules," and the 

 leaf is said to be " stipulate." Those hitherto described had none and are, 

 therefore, " exstipulate." Now what are "stipules " ? They represent a 

 basal pair of leaflets in compound leaves, and detached basal portions of 

 a single leaf ; but instead of growing out of the petiole as leaflets, as, 

 indeed, some stipules do, such as those of the Rose (though, the stipules 

 having the new function of protecting the bud in the " axil," they do not 

 assume the form of leaflets in the Rose), stipules usually arise directly from 

 the stem, the connection loith the petiole of the leaf being concealed within 

 the stem itself. The rule is that three, five, or more " fibro-vascular 

 cords " pass out of the cylinder of wood within the stem and enter the 

 petiole of the leaf. Then, secondary cords branch off from the two lateral 



