Why can grass be mowed?

It is obvious that grass is different from other plants because you can mow grass but cannot mow most other plants. What is the difference?

This is an interesting question related to plant anatomy. The grasses and many other plants called monocots differ from a separate group of plants called dicots in the manner in which growth occurs. The name monocot or dicot refers to the number of seed leaves (cotyledons) that develop when the seeds germinate. Monocots have a single cotyledon while dicots have two cotyledons. Compare the single leaf-like coleoptile that appears when corn germinates to the two "seed leaves" when a bean germinates. However, this is not the only difference.

The place in a plant where new cells are produced is called the meristem. The production of new cells along with the enlargement of the new cells is the source of growth in size of a plant. The meristem of many monocots is in the "crown" of the plant right at the soil level. (In a palm, it is at the base of the leaf cluster at the top of the trunk.) The meristem of dicots is at the end of each shoot. (Roots of both dicots and monocots have meristematic regions at the end of each root.) Because the meristem is at the end of the twigs and branches in a dicot, if you "mow" the dicot, you cut off the source of growth and new growing points must emerge from buds along the stem. Dormant buds contain inactive meristems that begin growth when the buds above it are removed. In monocots, since the growing point (meristem) is at ground level, you can cut the tips of the leaves and the meristem just continues producing new cells which enlarge and push the leaf blade up from the bottom. Therefore, you can mow the monocot grass but not the dicot.

This also explains the reason for careful pruning of dicots so that new growth from dormant buds is activated in the desired manner. This is a very limited discussion of a much more complex process, but I hope it helps you have a little better understanding of why you can mow grasses but not roses.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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