The question of whether trees “cry” arises from seeing liquid on their surfaces, but trees lack the nervous system or emotional capacity to weep. The visible droplets are not expressions of sadness, but the result of complex biological processes. These liquids serve specific functions, such as defense, repair, and water regulation, governed by chemical and physical forces. Understanding the science behind these secretions reveals a sophisticated biological strategy.
How Trees Sense and Communicate
While trees do not have a brain, they utilize sophisticated internal systems to recognize injury and communicate stress. Detection begins with damage-associated molecular patterns (DAMPs), molecules released upon tissue damage. Once injury is detected, the tree initiates rapid internal and external signaling.
Trees transmit information quickly through both chemical and electrical means. An injury triggers an electrical impulse that travels through the plant body, alerting distant leaves or roots to the danger. Simultaneously, the tree releases airborne chemical signals known as volatile organic compounds (VOCs).
These VOCs, such as methyl jasmonate, act as an alarm system. When an insect attacks, the damaged tree releases this chemical, prompting nearby plants to boost their defenses. This communication allows the tree to prepare for potential threats.
The defensive response is regulated by internal hormones, such as jasmonic acid. These hormones travel systemically through the vascular system, activating defense genes far from the injury site. This signaling helps the tree mobilize resources to repel attackers or repair damage.
Physical Responses to Damage
When a tree suffers a mechanical injury, the resulting liquid is a direct, defensive reaction to seal the wound and prevent infection. This visible secretion is often mistaken for a tree “crying.” The liquid’s chemical composition varies significantly depending on the tree species and its defense strategy.
Resin
In conifers, such as pines and firs, the defense involves the rapid production of resin, a thick, sticky substance composed mainly of terpenes. Resin serves multiple purposes: it physically seals the wound, traps invading insects, and acts as an antiseptic to prevent pathogen entry. This resin eventually hardens into a protective scab.
Gummosis
Broadleaf trees, particularly stone fruit trees in the genus Prunus (like cherries and peaches), respond to injury by producing a sticky liquid called gum, a process known as gummosis. This gum is a complex polysaccharide that oozes from the wound site, plugging the injury to block pathogen entry. Gummosis often signals stress from insect attack, damage, or fungal infection.
Sap Bleeding
A third type of liquid seen after injury is sap bleeding, the clear, watery flow of xylem sap from a cut. This is common in trees like maples and birches, especially in early spring. The flow is driven by internal pressure, pushing water and dissolved sugars out of the severed vessels. While not a dedicated defense substance, bleeding helps flush out potential pathogens.
The Phenomenon of Guttation
Another type of liquid droplet seen on plants is not a response to injury but a physical process called guttation. This phenomenon occurs when water droplets form on the tips or margins of intact leaves, often in the early morning. Guttation is unrelated to defensive secretions like sap, resin, or gum.
Guttation is driven by positive root pressure, which builds up when soil moisture is high and the air is saturated, preventing transpiration. Since the plant’s stomata are typically closed at night, the roots continue to absorb water, increasing internal pressure within the xylem vessels.
This pressure forces the water, containing dissolved minerals and salts, to exude through specialized pores called hydathodes. These small openings are located at the ends of the leaf veins. Guttation is a controlled release of excess water to manage internal pressure when the plant cannot transpire effectively.
The liquid from guttation is a dilute solution known as xylem sap, which sometimes leaves a white, mineral residue after the water evaporates. This process is most noticeable in small plants like grasses, but it also occurs in young trees and demonstrates the plant’s active regulation of its water balance.