308 



HUTCHINSON'S POPULAR BOTANY 



less highly developed condi- 

 tion, are found on the leaves 

 and young twigs of the Com- 

 mon Elm (Ulmus campestris}. 

 They are abundant along the 



j^t pr ^^^fr: midrib on the lower surface, 



" f^\ / - ' an d though they have nothing 



M' I . "^^ 1|| like the malignity of the Nettle, 



^rfjpy *'". ^i^fMi ^ej canse a considerable 



^8= LV^. 1 ^ amount of irritation to the 



hands and wrists of those who 

 touch the leaves. The Elm 

 sends up numerous suckers, 



and all down the bole it t } irows 



out new shoots, which would 

 be probably browsed off but 

 for the presence of these hairs. 

 The Elm belongs to the same 

 natural order (Urticacese) as the 

 Nettle. 



When investigation has 

 been carried farther it will be 

 shown probably that the hair- 

 structures of many other plants 

 have a protective purpose. 

 Many species of Mullein (Ver- 

 bascum), for example, have 

 branched radiating hairs which rub off easily when the plant is handled, 

 and, though not stiff or prickly, "remain hanging to the smallest in- 

 equalities on the surface of the disturbing body. If grazing animals 

 bring the mucous membrane of their mouths into contact with the leaves 

 of the Mullein," the Hock-like masses of hair adhere to the tongue and 

 palate and produce sensations that can hardly be pleasant (Kerner). The 

 hairs of several species of Primula, also (notably P. obconica), set up an 

 inflammation very like eczema when handled incautiously. 



It may not be generally known that the singular growths called galls, 

 so often to be seen on the leaves and branches of Flowering Plants, are 

 due to insects. Formerly they were held to be entirely of a vegetable 

 nature, and the insects found in them were thought to have been spon- 

 taneously generated there. Many species of Cynips lay their eggs in the 

 parts named, plunging their exceedingly delicate ovipositors into the soft 

 tissues, and thereby set up local irritation, which induces a responsive 

 action of the protoplasm, and galls are produced. In some cases, how- 

 ever, the development of the gall does not commence until after the egg is 



Photo by} 



FIG. 375. SPIKED PEA-GALL, 



On Dog-rose, caused by the gall-wasp, Rhodite, 



