Xanthium pennsylvanicum

Floral initiation is best studied at the microscopic level. The fact is, floral initiation often occurs and is unrecorded because abortion takes place or there is failure of the flower initials to develop to the macroscopic level where detection is easy with the unaided eye. There are many pieces of evidence that this is true. One was provided by Naylor (1941) with the photoperiodically sensitive cocklebur plant (Xanthium pennsylvanicum). He found that even when the plants were severely deficient in nitrogen or phosphorus or potassium they differentiated floral primordia after the leaves received an appropriate photoperiodic treatment. However, floral development did not occur. If dissection of stem apices had not been carried out, floral initiation would have gone undetected and the plants would have been recorded as vegetative. 

Neidle EK (1939) Nitrogen nutrition in relation to photoperiodism in Xanthium pennsylvanicum. Bot Gaz 100 607-618... 

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