One alkaloid is found in many genera of the loganiaceæ; berberine in genera of the berberidaceæ, ranunculaceæ, menispermaceæ, rutaceæ, papaveraceæ, anonaceæ.
Waxes are widely distributed in plants. They occur in quantities in some closely related families.
Ethereal oils occur in many families, in the bark, root, wood, leaf, flower, and fruit; particularly in myrtaceæ, laurineæ, cyperaceæ, crucifereæ, aurantiaceæ, labiatæ, and umbelliferæ.
Resins are found in most of the higher plants. Tropical plants are richer in resins than those of cold climates.
Chemical resemblance between groups, as indicating morphological relations, has been well shown. For example: the similarity[37] of the viscid juices, and a like taste and smell, among cactaceæ and portulaceæ, indicate a closer relationship between these two orders than botanical classification would perhaps allow. This fact was corroborated by the discovery of irritable stamens in Portulaca and Opuntia, and other genera of cactaceæ.
Darwin[38] states that in the compositæ the ray florets are more poisonous than the disk florets, in the ratio of about 3 to 2.
Comparing the cycadeæ and palmæ, the former are differently placed by different botanists, but the general resemblance is remarkable, and they both yield sago.
Chemical constituents of plants are found in varying quantities during stated periods of the year. Certain compounds present at one stage of growth are absent at another. Many facts could be brought forward to show the different chemical composition of plants in different stages of growth. The Thuja occidentalis[39] in the juvenescent and adult form, offers an example where morphological and chemical differences go hand in hand. Analyses of this plant under both conditions show a striking difference.
Different parts of plants may contain distinct chemical compounds, and the comparative chemical study of plant orders comprises the analysis of all parts of plants of different species.
For example; four portions of the Yucca angustifolia[40] were examined chemically; the bark and wood of the root and the base and blades of the leaves. Fixed oils were separated from each part. These were not identical; two were fluid at ordinary temperature, and two were solid. Their melting and solidifying points were not the same.