Xylem is one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients.
Crossing over is a process that happens between homologous chromosomes in order to increase genetic diversity. During crossing over, part of one chromosome is exchanged with another. The result is a hybrid chromosome with a unique pattern of genetic material.
Your answer would be photosynthesis because photosynthesis synthesis carbon based energy molecules from chloroplast inside a cell, with the outer membrance.
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The sun is the source for photosynthesis