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.