The Zimmerman Note was a note sent from Germany to Mexico asking Mexico to align itself with German interests. If the US entered the war and if Mexico opposed the US, the reward would be Arizona California and one other state -- perhaps Texas. It was a major contributor to the declaration of war on Germany by America.
The sinking of the Lusitania was another major event. When the Lusitania was sunk, 2000 people were affected.
Submarine warfare really aggravated the US. Many merchant ships were sunk.
Those specific plants were having significantly high death rates to probably something eating them so overtime they evolved defenses to make them less desirable.
Explanation:
So, just a problem with predators to make it simple. They needed to evolve to better survive.
The exchange introduced a wide range of new calorically rich staple crops to the Old World—namely potatoes, sweet potatoes, maize, and cassava. The primary benefit of the New World staples was that they could be grown in Old World climates that were unsuitable for the cultivation of Old World staples.
