Spot, a disease occurring on the leaves of the pelargonium, is a dry gangrene, occasioned by an irregularity in the supply of moisture and vicissitudes of temperature, but especially if one of the extremes is much below the degree of heat must favourable to the healthy growth of that plant. The reason of this is very obvious. If a pelargonium, or any other plant, be placed in a highly stimulating heat, and is abundantly supplied with root moisture, it immediately increases its surface of leaf to elaborate and digest the large amount of sap forwarded from the roots. If this amount of sap is subsequently suddenly reduced, by lowering the temperature and adding water to the soil less freely, the increased surface of leaf is no longer required, and it is a law pervading all the vegetable creation that the moment any of the parts of a plant are unnecessary to it, that moment it begins to decay. I placed a plant of the Marvel of Peru, or Heliotrope, in a high temperature and abundant moisture; these were then much reduced, and the leaves in a few clays were completely decayed round their edges, and in spots upon their surfaces.

The extent of leaf was accommodated to the amount of sap to be elaborated. - Princ. of Gard.