xxxvi.] POTATO DISEASE, II. PASSIVE STATE. 319 



plant grown in crowded positions, to a large production of 

 tubers. He thinks, in agreement with suggestions often 

 made by writers on horticultural subjects, that the absence 

 of flowers and berries on cultivated potatoes is a proof 

 that the plant is in an unnatural and disorganised con- 

 dition. Mr. T. A. Knight has shown, in the Philosophical 

 Transactions for 1806, p. 297, that the varieties of pota- 

 toes which uniformly produce neither flowers or berries 

 may be caused to produce them by preventing the growth 

 of tubers and runners amongst the fibrous roots. 



The writings of Mr. J. L. Jensen of Copenhagen, as 

 laid before the horticultural world by Mr. C. B. Plow- 

 right, M.R.C.S., have chiefly had reference to what Mr. 

 Jensen has termed " Protective Moulding." 



It is a common practice amongst potato growers to 

 earth-up potatoes, usually by driving a plough between 

 the rows. This earthing-up not only helps to support 

 the potato haulms in an upright position in the rows, but 

 it keeps the potato tubers from the light and consequent 

 greening ; it obviously keeps the potatoes free from the 

 numerous injuries they sometimes sustain from the attacks 

 of wire-worms, slugs, snails, rabbits, rats, moles, and other 

 animals, and from cracking after exposure to sun, hail, rain, 

 and wind. A potato when scratched, bitten, or bruised, 

 and with its inner substance exposed, is much more liable 

 to the attacks of fungus parasites than examples with the 

 natural armour of a perfectly whole skin. We have 

 shown that the mycelium of the fungus enters the potato 

 plant by the organs of transpiration, and sometimes even 

 pierces the epidermis or bark in its effort to reach the 

 interior. It follows, then, that any injury to the leaves, 

 stem, or tuber, even if the injuries are of the most micro- 

 scopic proportions, must aid the parasite in its efforts to 

 gain access to the inner tissues of the host. The fact has 

 long been accepted by a large section of potato growers 

 that earthing-up has also a marked tendency to keep 

 potato tubers free from the murrain, even when the 



