SECRETIVE TISSUES. 145 



resemblance between tumours and galls, tbongh originating 

 from different sources, both being liypertrophied conditions 

 of certain normal tissues. For example, Sir B. C. Brodie 

 thus describes a fatty tumour : " There is no distinct 

 boundary to it, and you cannot say where the natural adipose 

 structure ends and the morbid growth begins." It is pre- 

 cisely similar with galls, which are due to cell-division 

 setting in at certain points of the epidermis and subjacent 

 tissues. 



Although lesions and mutilations will not as a rule prove 

 to have any hereditary effects, yet the tendency to respond to 

 an irritation becomes permanent, and the form and structure 

 of the resulting organ may actually appear long before the 

 irritation is applied. This is conspicuously the case in the 

 tendrils of Ampelopsis Veitchii, in which the adhesive "pads" 

 are in preparation before any contact with a wall has taken 

 place. This is not the case with A. hederacea. Similarly the 

 aerial and climbing roots of Ivy are regularly produced 

 only on the shaded side. They can, however, be readily 

 made to form on the opposite side, if that be artificially 

 shaded ; and where, indeed, they may be not infrequently 

 found in nature, where they can be of no use. Such cases 

 prove that the tendency to produce them is an hereditary 

 affection which is present before the irritation is brought into 

 play. Again, with regard to the tendrils of the Oucurbitacece, 

 though the coiling does not take place till the irritating effect 

 induced by contact with a foreign body has brought it about, 

 yet the tendency is seemingly so strongly hereditary that, 

 in several cases, the tendrils are coiled while undeveloped in 

 the bud, and have to straighten themselves before again 

 coiling on contact, as may be seen in the common Bryony. 



In the case, however, of a mutilation, when it has been 

 once made, the place heals over, and there is an end of all 

 15 



