Across the world there may be three trillion trees. A mature tree may have 200,000 leaves, so there are a lot of leaves in the world - not counting those on herbaceous plants, grasses and shrubs.  The broad structure of a leaf is outlined here in woodlands.co.uk Tree ID. The leaf is the site of photosynthesis, providing food for the tree, and oxygen for us.  As the leaf is rich in nutrients, it is a source of nutrition for many organisms - other than the tree.  Oak trees are said to support over 2000 species, ranging from mammals, birds, beetles, spiders, fungi - through leaf-based food chains.  Leaves also support many micro-organisms through the detrital food chain (the decomposition of leaves in the litter layer and the soil).

We do not eat many tree leaves, though some do make their way into our diet.  For example, the evergreen shrub Camellia sinensis is widely grown in many parts of the world for the production of tea.  The young leaves can be picked in spring and dried to make tea.  Leaves of other plants are used in various herbal infusions or for flavouring such as bay, sage, oregano, thyme etc.

The fact that leaves are attractive to so many herbivores means that trees (and other plants) take measures to protect themselves. Some measures are physical - such as spines, thorns, prickles etc.   But when is a thorn a thorn, rather than a spine or a prickle?  These terms are used casually and interchangeably. 

Botanically speaking, they are all ‘spinose structures’ that is hard, rigid extensions or modifications of leaves, roots, or stems - all of which have sharp, stiff ends and all have the same role - to deter animals from eating the plant that bears them.  Plants that bear sharp structures that deter herbivory are termed spinescent.  There are differences between these various ‘structures’.

• thorns are derived from shoots (they may be branched or not, may or may not have leaves). The thorns of Hawthorn (Crataegus monogyna) can bear leaves.
• spines are derived from leaves (they may be formed from all of the leaf or just part of it and like thorns they have vascular tissue*)
• prickles are derived from the epidermis (the outer layer of cells of a stem, root or a leaf).   Prickles may be found almost anywhere on a plant and they do not have vascular tissue inside. 

Wild lemon and lime trees (Genus: Citrus) have spines, which protect young plants and indeed the fruits. The defences on roses are often described as thorns, but they are prickles, as they do not have vascular tissue (xylem and phloem) inside them. Sometimes, the leaf epidermis forms smaller, ‘simpler’ physical barriers called trichomes.  These are outgrowths of epidermal tissue but generally consist of only a few cells which form a defence against small insects.  Equally, a thick,  waxy cuticle on a leaf may be something of a deterrent to smaller insects.

Leaves sometimes form ‘teeth’ on the leaf margins and leaf apices.  A classic example of this is seen in Holly.  Holly leaves that develop at ground level are wavy, with large triangular ‘teeth’, bearing spines.   As the tree grows and holly can reach up to 80 feet,  the leaves become less spiny. The spines offer protection against grazing animals at the lower levels but are no longer needed when the trees reach a certain height.

While physical defences such as spines, prickles and trichomes can deter various herbivores,  chemical defences may also be deployed.  Chemical defences can take different ‘forms’, such as 

• 

  Oozing latex - Euphorbia

  tannins and phenolics. These create an bitter taste, they are complex polyphenols built from several phenolic molecules. Tannins are common in leaf tissues - particularly in the cells on the top surface of a leaf.  Scale leaves of buds are often particularly rich in tannins, reducing  the palatability or "tastiness" of the tissue thereby offering protection from herbivores. 

• Alkaloids are again usually bitter tasting compounds -, many of them derived from amino acids.
• Glycosides, as the name suggests, contain a sugar that is joined to another chemical, such as cyanide (as seen in bitter almonds (amygdalin).

Another possibility is that leaves may emit chemicals (aka VOC’ volatile organic compounds, scents, aromas) that deter insect visitors, or if a leaf is under attack by a insect pest then a leaf may release a VOC to ‘warn’ nearby plants of the attack so that they produce chemicals that make the leaves distasteful.

How long a leaf lives is incredibly variable, it may be eaten within days of its formation, it may last till autumn or it may last for years.  Many trees of temperate climes are deciduous, that is they shed their leaves come the shorter days of autumn.  The advantage of this is that the tree offers less resistance to the winds of winter, so is less likely to suffer physical damage (also true of snowfall).  The tree enters a state of dormancy until spring.  If in spring the tree produces flowers before the leaves (like Blackthorn) this  can facilitate wind dispersal of the pollen. 

However, losing leaves each year means that their nutrients are either lost or have to be moved out and stored somewhere else.  Having longer lasting leaves means that nutrients are retained, which is a distinct advantage in a nutrient poor, harsh environment. 

The longest lived leaves are found in a plant of the Namib Desert : Welwitschia.  This plant has two leaves throughout its life of some two thousand years.  The leaves may reach a length of 4 metres, the ends die or get worn away but the base generates new tissue.

  Welwitschia is a type of Gymnosperm.

Image (with thanks) by Nhelia from Pixabay