40 SCIENCE PRIMERS. [x. 



66. The branching of trees forms a most admirable 

 winter study, and one full of interest. It is sufficient 

 to allude to the zigzag branchlets of the oak, with 

 rounded terminal buds ; the graceful, straight twigs of 

 the beech, with lancet-shaped buds ; the bold stout 

 branchlets of the horse-chestnut, with ovoid buds; and 

 the exquisite subdivided sprays of the elm, like lace- 

 work against the sky. All these characteristic fea- 

 tures are due to the form, direction, and setting on of 

 terminal twigs and buds, and are objects of equal 

 interest to the botanist and the artist. Leafless 

 branches of the common English trees suspended 

 against a white wall are capital studies for pupils, and 

 reveal characters that escape the observation of 

 teachers whose attention has not been called to them. 



X. LEAVES. 



67. Leaves are expansions of the cellular-tissue of 

 the stem traversed by nbro-vascular bundles (Sect. 9). 

 Their use is to afford a large surface for exposing to 

 the action of sun-light and heat the food absorbed 

 by the plant and thus cause assimilation (Par. 28) ; 

 they also provide for evaporation (Par. 27) ; and they 

 absorb the carbonic acid of the air (Pars. 26, 28). 



68. The external characters of leaves are very 

 various indeed, and are characteristic of whole groups 

 as well as of individual kinds of plants. The following 

 principal facts regarding the leaves of some common 

 plants should be observed by the pupil : 



(a) As to duration. Whether they are deciduous, 

 falling annually; or persistent, continuing a year or 

 longer. 



