_Sun-spots._--Yellow spots, which may turn brown or black according to the species of plant affected and the intensity of the action, are often caused by the focussing of the solar rays by lens-like thickenings due to inequalities in the glass of greenhouses, or by drops of water on them or on other leaves, _e.g._ Palms, _Dracaena_, etc. The action is that of a burning glass, and extends throughout the leaf-tissues. Young grapes, etc., may also be injured in this way. Water-drops on the glass can only act long enough to produce such injuries if the atmosphere is saturated. The old idea that a drop on a leaf can thus focus the sun's rays into the tissues beneath is not tenable. Here again we see that the disease-agencies concerned in producing the symptoms described in this chapter, agree for the most part in so far that the principal effect is generally the disturbance of chlorophyll action in the spots or flecks on the leaves, and the rendering useless of these areas so far as providing further food-supplies is concerned. The effects may be due merely to the shading action of a parasite--_e.g._ epiphytic fungi--or to actual destruction of the tissues invaded--_e.g._ by endophytic fungi--or the tissues may be burnt, poisoned, etc. In so far the results are again quantitative and cumulative, and the amount of damage depends on the number and size of the spots or other areas affected, and the proportion of foliage involved, as well as the length of time the injurious action is at work. But, again, it must be remembered that several symptoms may co-exist, and matters may be complicated by the spread of the destructive agent, or its consequences, to other parts, and in some cases we are quite uninformed as to the true nature of the disease. NOTES TO CHAPTER XX. Further information regarding these "leaf-diseases" will be found in special works dealing with the fungi and insects which cause them. In addition to works already quoted, the reader may also be referred for Fungi to Massee, _A Textbook of Plant-diseases caused by Cryptogamic Parasites_, London, 1899; or Prillieux, _Les Maladies des Plantes Agricoles_, 1895. See also Marshall Ward, Coffee-leaf Disease, _Sessional Papers_, XVII., Ceylon, 1881, and _Journ. Linn. Soc._, Vol. XIX., 1882, p. 299. The question of "Sun-spots" has been dealt with by Jönnson in _Zeitschr. f. Pflanzenkrankh._, 1892, p. 358. CHAPTER XXI. ARTIFICIAL WOUNDS. _The nature of wounds and of healing processes--Knife wounds-- Simple cuts--Stripping--Cuttings--Branch-stumps and pruning-- Stool-stumps--Ringing--Bruises._