THE LEAF IN RELATION TO ITS ENVIRONMENT 



309 



hatched. No tree, probably, furnishes so great a variety of these growths 

 as the Oak (Quercus), on which upwards of one hundred and fifty species have 

 been observed, the well-known oak-apple being one of them. It is pro- 

 duced by the punctures of Dryoteras tenninalis. Two species of Oak-gall 

 often to be met with are produced by an insect named Spathegaster bac- 

 carum. Fig. 373 represents a leaf of a species of Lime (TUia platyphyllos) 

 with little conical excrescences or nail-galls, the work of a microscopic 

 species of Phytoptus (P. tilce), whose portrait you will notice just above 

 the leaf. Similar galls, but of a downy nature, occur on the leaves of 

 Beech (Fagus sylvatica], in this case caused by a two-winged fly (Hormomyia 

 piliger). The green (ultimately red), mossy-looking growths called bede- 

 guars, or Robin Redbreast's Pincushion, so common on branches of the 

 Rose, are also of insect origin (fig. 371). The gall-fly (Rhodites rosea) 

 deposits its eggs in the shoot-bud, which presently swells and begins to 

 put out what should be in the natural course three leaves : but embryo 

 leaves have so little parenchyma between their veins that they fall into 

 threads and thus give the mossy appearance to the galls. On cutting 

 open one of the bedeguars the larvae of the insect will be found in the 

 centre. In all these cases it appears that only the cells of the meristem can 

 give rise to the galls. These growths do not appear to be actually injurious. 



The galls that are 

 found on Hedge Bed- 

 straw (Galium mollugo) 

 are produced by a 

 minute two-winged fly 

 (Cecidomyia aparine) ; 

 and the many-cham- 

 bered gall on young 

 shoots of Spruce (Picea 

 excelsa), that look like 

 half a pine cone, are 

 the work of a Coccus 

 (Kermes). The "Witches 

 Broom " on the Scots 

 Pine (Pimis sylvestris) is 

 an excrescence consist- 

 ing of a multitude of 

 short shoots produced 

 by a fungus (Perider- 

 mium elatinum). 



What is known as 

 fasciation, or the fusing 

 together of parts of a 

 plant which are norm- 



Photo by] \- E - 



FIG. 376. "WITCHES BROOM " ON SCOTS PINE, 



Caused by the fungus Peridermium elatinum. 



