How does the north-south axis prevent diffusion of crops?This can be found in chapter 10

The idea here is that continents that have a long
north-south axis are worse than continents that have a long east-west axis when it comes
to having crops diffuse.  This is because similar latitudes have similar climates (all
other things being equal).  By contrast, different latitudes have different climates. 
This means that a crop that can thrive at one latitude can spread east to west along
that same latitude.  But if a continent has a north-south axis, it is much harder for
the crop to spread.  A crop that thrives near the equator in Colombia, for example, is
not that likely to do well in Argentina since the climate will be much different
there.