_Discoloured spots_ or patches on the herbaceous parts of plants, especially leaves, furnish the prominent symptoms in a large class of diseases, due to many different causes, and although we cannot maintain this group of symptoms sharply apart from the last, as seen from the considerations on _albinism_, it is often well marked and of great diagnostic value. By far the greater number of spot-diseases are due to fungi, but this is by no means always the case. The most generally useful method of subdividing the classes, though artificial like all such classifications, will be according to the colour of the spots or flecks, which, moreover, are usually found on the leaves. It is necessary to note, however, that various conditions may modify the colour of spots on leaves. Many fungi, for instance, induce different coloured spots according to the age of the leaf or other organ attacked, or according to the species of host, the weather, etc. Moreover the spots due to these parasites are frequently yellow when young and some other colour, especially brown or black, when older. _Scale_ is the name given to the characteristic shield-like insects (_Mytilaspis_, _Aspidiotus_, etc.) which attach themselves to branches of Apples, Pears, Oranges, Camellias, and numerous other plants, and suck the juices. It is the female insect which has the body broadened out into the "scale," under which the young are brought up. Enormous damage has been done by some forms--_e.g._ the San José scale in the United States. The superficial resemblances of the patches of eggs of some Lepidoptera to Aecidia and other fungi may be noted in passing--_e.g._ _Bombyx neustria_ on Apple twigs, _Aporia Crataegi_. _White_ or _greyish spots_ are the common symptom marking the presence of many Peronosporeae and Erysipheae in or on leaves, _e.g._ _Peronospora Trifoliorum_, _P. parasitica_ on Crucifers, etc., and _Sphaerotheca_ on Hops; also _Septoria piricola_, _Cystopus_, _Entyloma Ranunculi_, etc. White spots are also caused by insects such as _Tetranychus_ (red spider) on Clover and other plants. _Yellow_, or _Orange-coloured Spots_. In cases where these occur on leaves, and in the case of grasses, etc., on the leaf sheaths as well, they commonly indicate the presence of Uredineae, and sections under the microscope will show the mycelium in the tissues beneath. Species of _Uromyces_, _Puccinia_, etc., in the Uredo state have the spots powdery with spores; _Aecidia_ show the characteristic "cluster cups," and so forth. These spots are often slightly pustular, and in some cases markedly so. Other fungi also induce yellow spots on leaves--_e.g._ _Phyllosticta_ on Beans, _Exoascus_ on Poplars, _Clasterosporium_ on Apricot leaves, _Synchytrium Succisae_ on _Centaurea_, etc. Yellow spots are also a frequent symptom of the presence of Aphides, of Red Spider, etc. Thus the minute golden yellow spots sometimes crowded on Oak leaves are due to _Phylloxera_ punctures.