Characteristic examples of homomorphism are seen in the resemblances which many species of Euphorbia present to the cactus tribe and in the pollen-masses of the orchids and asclepias. In Britain the order Euphorbiaceæ is represented by the box, dog’s-mercury, and the sun-spurges, but many foreign species have quite a different appearance and agree with the cacti in their aborted leaves and green succulent stems. The globular, columnar, and angular forms give to both a peculiar aspect by which they are broadly distinguished from all other vegetable types; and yet in systematic position these two orders stand far apart. The nearest affinities of the Euphorbiæ are with the Urticaceæ and other orders having incomplete flowers, while the nearest allies of the Cacti are the Cucurbitaceæ and other calycifloral orders. Succulent stemmed plants of this description are specially adapted to an arid climate, and it is not unreasonable to suppose that the similarity between the Euphorbiæ and Cacti results from the long-continued action of similar external conditions upon similarly endowed tissues. The Australian Casuarinas are dicotyledons with incomplete flowers nearly related to the oak, hazel, and other Cupuliferæ, but in outward appearance they have a singular resemblance to the horsetails, a family of cryptogams. One of the gymosperms or cone-bearing class, Ephedra, also presents the same jointed appearance so characteristic of Equisetaceæ. Growing in marshy places very like those affected by Equisetum we find the mare’s-tail Hippurus, a flowering plant allied to the fuchsia family, but externally resembling Equisetum in its jointed stem and whorled leaves. A familiar instance of the same kind of homomorphism is Equisetum sylvaticum, which might almost be described as a liliputian fir-tree. The little flowers of the water ranunculus look exactly like miniature water lilies, while the leaves and flowers of Caltha palustris simulate the yellow Nuphar so much that in some parts of the country the marsh marigold is known as the water lily. The specific name of another aquatic, Lymnanthemum nymphædides, indicates a peculiarity of the same kind. Leaf analogies are frequent among aquatic plants; the orbicular, peltate leaf of the Indian cress occurs, for example, in Hydrocotyle, Nelumbium, and others. The brown color and translucence of Potamogeton, Myriophyllum, and other aquatics assimilates them to the fronds of Laminaria and other sea-weeds.