828b
DE PLANTIS
leaves before their fruits. In the case of plants which have slender parts the colour of the flower will resemble a bright blue; when the parts arc not closely compressed, it will tend to whiteness; under medium conditions it will be a blue-grey. The absence of flowers in certain plants is usually due to the variety of their parts and their rarity or their roughness or thickness. The palm and similar trees therefore have no flowers.
829a A plant which has thick bark expands owing to the pressure of moisture and the impelling force of heat; we see this in the pine and palm. A plant which gives forth a milky juice will have such juice within it; there will be powerful heat within and an oily substance will be present there. When the heat begins to cause assimilation, the oily substance will be turned into moisture, and the heat will solidify it to a slight extent, and local warmth will be caused, and an oily liquid will be produced similar to milk, and vapour will rise from the moisture which attracts the milky substance into the extremities of the plant, and the moisture will retain the heat[1] which appears. The milky substance will not be solidified, because it is the function of heat to solidify it.[2] If the milky substance shows any considerable degree of solidification, it will be due to the presence of cold in the tree. The milky substance will solidify when it has left its original position in the tree, and the result will be the formation of gum. Gum comes out warm from the tree by distillation, and, when it comes into contact with the air, it will solidify. Some gums flow in temperate places, and these will be of the consistency of water; others flow out and solidify as hard as stone or shell. Gum which flows drop by drop keeps its form, as in the tree which is known as aletafur.[3] The gum which changes into a stony substance will be very cold on its first appearance, and its appearance will be caused by heat, and when it flows it will turn to stone; it will occur where the soil is very hot. Some trees undergo a change in the winter and will become sometimes green and sometimes