154 HOUSE, GARDEN, AND FIELD 



fly. Nearly allied to this is the Hessian fly, which in some 

 countries is a serious pest to the wheat-crops. Of late years 

 the hawthorn-fly has been unusually mischievous in York- 

 shire. Earlier in the season, before the hawthorn-fly 

 begins its attack, a small caterpillar is often found in the 

 hawthorn-buds. 



There is more than one maple in the hedge. Besides 

 the sycamore-maple there is a smaller maple which rarely 

 grows higher than twenty feet ; this is the hedge-maple. 1 

 Gather leaves, flowers and fruits of each when you have 

 opportunity ; see also whether they have the same kind 

 of bark. (2) 



Take a green elder-shoot, snap it across, and draw the 

 ends gently apart ; as you do so you will pull out some 

 slender threads, which will lengthen to perhaps a couple 

 of inches before they break. What are these threads ? 

 The microscope or even a pocket-lens will tell you that 

 they are spiral vessels. Such vessels are characteristic of 

 young wood, and are generally found in the first- formed 

 bundles, close to the pith ; the later-formed vessels have 

 a different structure. Spiral vessels have the advantages 

 of great flexibility and great power of extension, both of 

 them important properties in a young and rapidly lengthen- 

 ing tissue ; at first they are filled with a watery fluid, but 

 in older stems they contain nothing but air. The analogy 

 between the spiral vessels of a plant and the air- tubes of 

 an insect is very close. 



An elder-shoot contains a great mass of pith, which at 

 first consists of small cells full of sap ; in a later stage the 

 cell-sap dries up, and the cells are filled with air. Of what 

 use can dead, air-filled cells be to the stem ? It is plainly 

 advantageous that the wood of the young elder-branches 

 should be placed far from the centre, and not collected in 

 one solid mass. The rigidity of a hollow cylinder is far 



1 Bentham calls it the common maple, but that name would not enable 

 most people to tell what species was meant ; in many parts of the country 

 the sycamore-maple is the commoner of the two. 



