Plant Wars

Mention the words "chemical warfare" or "deployed armies" in any conversation, and your interlocutor might immediately assume that you are talking about wars between humans. In reality, however, there are other kinds of wars in which these techniques are employed far more frequently and in a much more intricate manner: those waged in the plant kingdom.

We might not normally think of plants in this way, but much like humans and animals, they too have to fight for survival on a daily basis. Nutrients, light and water are the three things any plant needs in order to grow. Unfortunately, none of these is available in unlimited quantities, which means that competition between plants can become fierce.

Some plants and trees have an architectural advantage: taller trees have greater access to natural light, while plants with deeper roots are able to absorb more water and nutrients. Others, however, manage to defend their territory through "allelopathy", or chemical warfare.

So how exactly does this chemical warfare work? As Dr Robin Andrews explains, plants convert the nutrients they absorb from the ground into energy with the aid of a type of organic compound known as metabolites. These metabolites can be divided into two categories: primary and secondary.

Primary metabolites are what allow a plant to live, playing a direct role in its growth and development, and are therefore present in every plant. Secondary metabolites, on the other hand, can vary from plant to plant and often function as a defence mechanism against neighbouring competitors.

Among these secondary metabolites, there are two that are particularly interesting: DIBOA and DIMBOA. These two cyclic hydroxamic acids were at the forefront of a study conducted by Sascha Venturelli and colleagues in 2015. The study found that once they are released into the soil by the plants that produce them, they degrade into toxic substances that have the power to inhibit growth in nearby plants after being absorbed.

As Dr Claude Becker notes, "the phenomenon itself has been known for years", but we now finally understand the "molecular mechanism" behind it -- and its supreme intricacy would put to shame any chemical bombs created by humans.

However, plants do not just fight wars against other plants; chemical warfare also plays a role in their defence against herbivores. As Brent Mortensen of Iowa State University describes, plants "actively resist" attacks made by herbivores through qualitative and quantitative chemical defences.

What is the difference? Qualitative defences can be lethal even in small doses and are often employed to protect "young" or "tender leaves or seeds". They can also be recycled when they are no longer necessary.

Quantitative defences, in contrast, are effective only "in larger doses", but unlike qualitative defences, they can protect the plant against all herbivores. Quantitative defences are also not immediately lethal, as they usually lead to indigestion, pain, irritation of the mouth and throat, and inflammation or swelling of the skin.

And what about the "deployed armies" mentioned earlier? Chemical attacks are not the only way plants choose to defend themselves against herbivores. Some plants, such as the African acacia, also recruit armies to assist them in their war.

As Angela White of the University of Sheffield explains, the acacia tree has "hollowed-out structures" that invite ant colonies to build homes in them by providing not only shelter, but also food in the form of special nectar. In return, the ants protect the trees against herbivores -- including not only small ones such as insects, but also animals as large as giraffes.

At this point, of course, you might be wondering what all this has to do with you. The territorial nature of plants might be fascinating in its own right, but what is its application in real life?

Dr Venturelli, from the 2015 study mentioned earlier, has an answer: apparently, certain allelochemicals -- the aforementioned chemical compounds responsible for stunting growth in plants -- have been found to have an effect on human cancer cells as well.

According to Michael Bitzer and Ulrich Lauer, who participated in the same study, "clinical trials at the University Clinics Tübingen currently assess the efficacy of these plant toxins in cancer patients". This means that understanding how plants defend themselves against enemies in their environment might be of interest not only to plant biologists, but also to medical researchers.

Investigating Children’s Language

A For over 200 years, there has been an interest in the way children learn to speak and understand their first language. Scholars carried out several small-scale studies, especially towards the end of the 19th century, using data recorded in parental diaries.

However, detailed, systematic investigation did not begin until the middle decades of the 20th century, when the tape recorder came into routine use. This made it possible to keep a permanent record of samples of children’s speech so that analysts could listen repeatedly to obscure extracts and thus produce detailed and accurate descriptions.

Since then, the subject has attracted enormous multidisciplinary interest, notably from linguists and psychologists, who have used a variety of observational and experimental techniques to study the process of language acquisition in depth.

B Central to the success of this rapidly emerging field is the ability of researchers to devise satisfactory methods for eliciting linguistic data from children. The problems that have to be faced are quite different from those encountered when working with adults. Many of the linguist’s routine techniques of enquiry cannot be used with children.

It is not possible to carry out certain kinds of experiments because aspects of children’s cognitive development -- such as their ability to pay attention or remember instructions -- may not be sufficiently advanced. Nor is it easy to get children to make systematic judgements about language, a task that is virtually impossible below the age of three.

Anyone who has tried to obtain even the most basic kind of data -- a tape recording of a representative sample of a child’s speech -- knows how frustrating this can be. Some children, it seems, are innately programmed to switch off as soon as they notice a tape recorder being switched on.

C Since the 1960s, however, several sophisticated recording techniques and experimental designs have been devised. Children can be observed and recorded through one-way-vision windows or by using radio microphones, so that the effects of having an investigator in the same room as the child can be eliminated.

Large-scale sampling programmes have been carried out, with children sometimes being recorded for several years. Particular attention has been paid to devising experimental techniques that fall well within a child’s intellectual level and social experience.

Even pre-linguistic infants have been included in the research: acoustic techniques are used to analyse their vocalisations, and their ability to perceive the world around them is monitored using special recording equipment. The result has been a growing body of reliable data on the stages of language acquisition from birth until puberty.

D There is no single way of studying children’s language. Linguistics and psychology have each brought their own approaches to the subject, and many variations have been introduced to cope with the variety of activities in which children engage and the wide age range they represent. Two main research paradigms are found.

E One of these is known as "naturalistic sampling". A sample of a child’s spontaneous use of language is recorded in familiar and comfortable surroundings.

One of the best places to make the recording is in the child’s own home, but it is not always easy to maintain good acoustic quality, and the presence of the researcher or recording equipment can be a distraction, especially if the proceedings are being filmed.

Alternatively, the recording can be made in a research centre, where the child is allowed to play freely with toys while talking to parents or other children, and where the observers and their equipment are unobtrusive.

F A good-quality, representative, naturalistic sample is generally considered an ideal datum for child-language study. However, the method has several limitations. These samples are informative about speech production, but they provide little guidance about children’s comprehension of what they hear around them.

Moreover, samples cannot contain everything, and they can easily miss important features of a child’s linguistic ability. They may also not provide enough instances of a developing feature to enable the analyst to reach a decision about the way the child is learning. For such reasons, descriptions of samples of children’s speech have to be supplemented by other methods.

G The other main approach is through experimentation, and the methods of experimental psychology have been widely applied to child-language research.

The investigator formulates a specific hypothesis about children’s ability to use or understand an aspect of language and devises a relevant task for a group of subjects to undertake. A statistical analysis is made of the subjects’ behaviour, and the results provide evidence that either supports or falsifies the original hypothesis.

H Using this approach, as well as other methods of controlled observation, researchers have produced many detailed findings about language production and comprehension among groups of children. However, it is not easy to generalise from the findings of these studies.

What may occur in a carefully controlled setting may not apply in the rush of daily interaction. Different kinds of subjects, experimental situations and statistical procedures may produce different results or interpretations.

Experimental research is therefore a slow, painstaking process. It may take years before researchers are convinced that all the variables have been considered and that a finding is genuine.