74 THE LEAF 



(e) Leaf-spines. —In the sloe and other shrubs and trees certain 

 branches are found which have been modified into short, stiff 

 spines. That the latter are branches or shoots is seen from the 

 fact that they frequently bear small leaves and buds. 



In some plants however, such as barberry, the spines are 

 evidently not branches, but modified leaves, for buds and stems 

 frequently appear in their axils, and in the former plant all 

 stages of transition between an ordinary green leaf and a 

 branched spine are frequently observable on the same plant. 



(/) Leaf-tendrils. — In the vetch and pea (Fig. 33) the terminal 

 leaflets, instead of being green, are modified into thin, thread- 

 like structures termed tendrils. They are sensitive to contact 

 and wind round any small object which they touch. 



In some plants, such as the vine and passion-flower, the 

 tendrils are not leaves but modified branches. 



Ex. 37. — Examine the cotyledons of the seedlings of weeds springing up 

 on garden soils and arable ground. Note the difference between these and 

 the first foliage-leaves. 



Examine the cotyledons of seedlings of the common farm crops. 



Ex. 38. — Examine the scales of an onion, tulip, and lily bulb, and those 

 upon the underground stems of couch-grass and other plants. 



Ex. 39. — Examine the spines on a gooseberry bush. Do they belong to 

 the leaves or are they modified shoots ? 



Note both the leaf-spines and stem-spines upon ordinary gorse. 



Compare with Ex. 29. 



Ex. 40. — Note the form and position of the tendrils of a vetch and pea, 

 both when free and when wound round a support. 



4. Leaf-arrangement. — Although to a casual observer the 

 leaves appear to be without any regular arrangement upon a 

 plant, careful inspection shows that they are distributed on the 

 stems in a very definite order, which is usually constant for each 

 species. 



In some, such as the sycamore (Fig. 14), dead-nettle, and 

 cleavers, two or more leaves arise at the same node of the stem. 

 Each collection of leaves is then called a whorl, and the 



