42 SCIENCE PRIMERS. [x. 



be employed as exercises for the observation of the 

 beginner, who should be able to apply all those terms 

 mentioned above to the trees and shrubs of his 

 county. 



69. The way in which leaves are folded together, 

 or doubled up, or rolled up in bud, is called their 

 vernation, and is an excellent subject for the pupil's 

 observation. Thus, in grasses and in the cherry they 

 are simply rolled round one another ; in the apple 

 they overlap in opposite pairs ; in the flag they are 

 sharply folded upon one another ; in ferns they are 

 rolled inwards from the top like a crozier \ in the 

 pear and apple the margins of each leaf are rolled 

 inwards, and in the rosemary and the willow the 

 margins are rolled backwards ; in the vine, beech, and 

 gooseberry, each leaf is plaited. 



70. The chief substance of all leaves is cellular 

 tissue (Par. 7), continuous with that of the stem. The 

 cellular tissue is traversed by fibre-vascular bundles 

 (Par. 9), also continuous with those of the stem. The 

 leaf tissues are thus, like those of the bud (Par. 62), 

 in complete union with those of the stem. 



71. A cross section of a leaf shows, beginning at 

 the upper surface (a\ a delicate skin (epidermis) 

 (Par. 6) of flattened transparent cells ; (b) a layer of 

 close packed cells full of green chlorophyll granules 

 (Par. 1 6); (c) several layers of loosely packed cells with 

 air-spaces between them ; (d] an epidermis similar to 

 that of the upper surface. 



The vascular bundles consist of bast- tissue towards 

 the under surface of the leaf, and wood-tissue, with 

 vascular tissue, usually consisting of spiral vessels, 

 towards the upper surface. 



