Unlike animals, plants do not have a digestive system, although the sustainable food trust makes a good argument that ‘soil is the collective stomach of all plants’ref Trees synthesise all of the substances they need to live and grow from 17 nutrients. It’s important to understand that plants don’t ‘eat food’ in the sense of consuming sugars, fats or proteins like animals do. Aside from oxygen, carbon and hydrogen (which come from air and water), trees absorb nutrients through their roots.
Water and nutrients are transported around trees via the xylem, a network of narrow dead cells which act like a kind of pipe. Nutrients are dissolved in the water (‘solutes’) and travel with it in the form of ions (charged molecules). To get into the xylem in the first place, water is absorbed into the root tips.
In many species this is done through the root hairs. Root hairs are “long tubular extensions of root epidermal cells that greatly increase the root surface area and thereby assist in water and nutrient absorption.”ref According to Thomas most live only for a few hours, days or weeks, and are constantly replaced by new ones as the root growing tip elongates. Some conifers do not have root hairs and rely on mycorrhiza instead to assist nutrient and water absorption.
In order to absorb water, the root tips need to be in physical contact with it, so having root hairs that reach into the soil provides contact with more water (and nutrients). Nutrients in the form of ions are ‘pumped’ into root hairs (or cells, if the species has no root hairs) using a process called active transport, which uses some of the energy from photosynthesis. Because the root cells have dissolved nutrients in them, water is then attracted into the space by osmosis.
From the roots tips, water and solutes make their way to the ‘stele’ – this is the central part of the root which contains the vascular system (xylem & phloem, shown in blue and red respectively in the left hand diagram below). Surrounding the stele is the endodermis – seen below in orangey-brown cells with red lines through them.
The red lines represent cells known as ‘Casparian strips’. They are full of lignin and other hydrophobic molecules, which basically plug any gaps between the endodermis cells. This forces any water or solute to pass through the endodermis cells. After this they travel through the root parenchyma cells into the xylem.ref
The existence of Casparian strips leads to a pretty important insight, which suggests that most molecules entering the xylem from the outside world are actively invited in, and have to be able to traverse a cell membrane. So the tree can theoretically control or at least limit what can enter. Vogel says “the sap that rises up a tree trunk has to be nearly free of dissolved material. So much water gets transpired that the accumulation of dissolved solids, coming out of solution as water evaporated in the leaves, would make big trouble as the growing season advanced.” So this implies there aren’t a lot of non-nutrients dissolved in xylem sap. But in fact, xylem has a microbiome (it’s part of the endosphere) and literally thousands of dissolved molecules in it (described more in xylem), so obviously the Casparian strips are not a 100% barrier.
It’s not all down to the root hairs or root tips though, symbiotic fungus known as mycorrhiza play an important role in enabling root function, read more about this in The Microbiome and Symbiotic Microbes.