Putting arrows (e.g., ↑,↓) into the axis labels of an R plot using expression

I was just working through the problem of trying to get up (↑) and down (↓) arrows into figure axis labels in R.

Just copying and pasting unicode arrows into the R script works for axis labels that are to be rendered by the R graphical device within an interactive R session.

Unfortunately, this does not work for figures or R markdown to be rendered as HTML (e.g., using knitr) or as PDFs (e.g., using rmarkdown)!

After digging online for some hints (the most useful one I found here, along with, of course, R help), the following is a better solution that uses the base function expression:

library(phytools)
tips<-getStates(ecomorph.tree,"tips")
tip.cols<-cols[tips]
plotTree.barplot(ecomorph.tree,scores(pca)[,3],
    args.plotTree=list(fsize=0.4),
    args.barplot=list(col=tip.cols,
    xlab=expression(paste("PC 3 (",""%up%"","limbs, ",
    ""%down%"","lamellae)",sep="")),
    cex.lab=0.8))
legend("topright",levels(anole.ecomorph[,1]),
    pch=22,pt.bg=cols,pt.cex=1.5,cex=0.9)

Here are the tree & data that I used:

ecomorph.tree

## 
## Phylogenetic tree with 82 tips and 81 internal nodes.
## 
## Tip labels:
##  ahli, allogus, rubribarbus, imias, sagrei, bremeri, ...
## 
## The tree includes a mapped, 6-state discrete character with states:
##  CG, GB, TC, TG, Tr, Tw
## 
## Rooted; includes branch lengths.

pca

## Phylogenetic pca
## Standard deviations:
##        PC1        PC2        PC3        PC4 
## 0.81378367 0.22553447 0.12277042 0.10577197 
##        PC5        PC6 
## 0.04920179 0.03691686 
## Loads:
##            PC1         PC2         PC3
## SVL -0.9712234  0.16067288  0.01972570
## HL  -0.9645111  0.16955087 -0.01203113
## HLL -0.9814164 -0.02674808  0.10315533
## FLL -0.9712265  0.17585694  0.10697935
## LAM -0.7810052  0.37429334 -0.47398703
## TL  -0.9014509 -0.42528918 -0.07614571
##             PC4         PC5         PC6
## SVL  0.14782215 -0.06211906  0.06935433
## HL   0.17994634  0.08064324 -0.04406887
## HLL -0.13790763  0.06887922  0.04126248
## FLL -0.09105747 -0.06075142 -0.04864769
## LAM -0.15871456  0.00217418  0.00875408
## TL   0.01709649 -0.01750404 -0.01088743

That's all for now.