背景知识1：
Weeping sap are plants displaying a natural defense against an insect infestation. Sap, also referred to as resin or sticky pitch, typically flows when insects attempt to enter the tree's bark（树皮）. The sap blocks the hole and often saturates and kills the insect。
Some defensive compounds are produced internally but are released onto the plant’s surface; for example, resins树脂, lignins木质素, silica硅, and wax cover the epidermis表皮 of terrestrial plants and alter the texture质地 of the plant tissue. The leaves of holly plants, for instance, are very smooth and slippery making feeding difficult. Some plants produce gummosis流胶 or sap that traps insects.
Mimicry模仿 and camouflage伪装：Some plants mimic the presence of insect eggs on their leaves, dissuading劝阻 insect species from laying their eggs there. Because female butterflies are less likely to lay their eggs on plants that already have butterfly eggs, some species of neotropical vines of the genus Passiflora (Passion flowers) contain physical structures resembling the yellow eggs of Heliconius butterflies on their leaves, which discourage oviposition by butterflies.[37]
背景知识2：
Sap is a fluid transported in xylem cells (tracheids or vessel elements) or phloem sieve tube elements of a plant. It transports water and nutrients throughout the plant. Fluid found in the vacuoles of other cells is sometimes referred to as "cell sap".[citation needed]
Xylem sap consists primarily of water, with hormones, mineral elements and other nutrients dissolved in it. Transport of sap in xylem is characterized by movement from the roots toward the leaves. Over the past century, there has been some controversy regarding the mechanism of xylem sap transport; today, most plant scientists agree that the cohesion-tension theory, along with evapotranspiration best explains this process.
Phloem sap consists primarily of water, with sugars, hormones, and mineral elements dissolved in it. It flows from where carbohydrates are produced or stored to where they are used. The pressure flow hypothesis proposes a mechanism for phloem sap transport.
Maple syrup is made from reduced sugar maple xylem sap. In some countries (e.g., Russia, Latvia, Estonia or Finland) it is common to collect the early spring sap of birch trees (so called "birch juice") for human consumption; the sap can be used fresh or fermented and contains xylitol