Endomycorrhizae.
   Hyphae  digest  small patches of the root cell walls, then grow into a
   tube  formed  by  invagination  of  the  cell's membrane, analogous to
   poking a finger into a balloon.
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   Within   the   root   cortex,  extensive  branching  of  hyphae  forms
   arbuscules, providing an enormous surface area for nutrient swapping.