Because they’re not red(!)

Although this may appear to be a smart-arse answer to the question ‘Why are plants green?’, it’s probably not too wide of the mark. Many interested parties have wrestled with the question and several suggestions have been made as to why most plants – by which commentators tend to mean the principal photosynthetic parts, the leaves – are predominantly green in colour. Most of these dwell on the preponderance of green-coloured chlorophylls (yes, plural – a and b) in land plants. See MinuteEarth’s charming video about this here, or ResearchGate’s academically-contributed thread on the issue here, or the undergraduate-student-targeted item by Mark McGinley (an Associate Professor in the Honors College and Department of Biological Sciences at Texas Tech University in Lubbock, USA) here. However, it seems that the ‘real’ answer rests with the evolutionary heritage of the land flora, as deduced by Jonas Collén et al. and their announcement of the sequencing of the genome of Chondrus crispus, a red alga/seaweed commonly called Irish moss.

Although red algae contain green chlorophyll, their red coloration is a result of large amounts of non-green pigments such as phycoerythrin. During the inferred course of its evolution, C. crispus lost many genes (its compact genome of 9606 genes compares with the unicellular green alga Chlamydomonas reinhardtii with 14 516 genes, and Arabidopsis thaliana’s 27 416). And this genetic reductionism would have had evolutionary knock-on effects. In particular, the loss of flagellar genes, needed for the motility of certain cells – especially the gametes during sexual reproduction in so-called ‘lower’ land plants  – may have been enough to have given ‘rival’ flagellate green algae the evolutionary ‘leg-up’ needed to allow them to claim the land as their own, and ultimately to beget the land flora. Or, as the paper’s press release puts it: ‘Had this massive gene loss never occurred, red algae might have extensively colonized the terrestrial environment, in the same way as green algae, which are the ancestors of all land plants’. And that’s why plants are green/aren’t red. ‘Just so!’, an exceedingly well-informed Mr Kipling is reported to have said.