How did scientists prove that the food is transported through the phloem

When a aphid is killing at the act of feeding then the body is carefull cut away but the proscis still inserted into the phloem then the food slowly exuded form cut end of the proboscis in the form of drops. These drops contain sugar and amino acids which means food is transported through phloem in plant.

then that particular organisms become extinct.

the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.