Please help 7th Grade class answer questions

jleipner at jleipner at
Tue Jun 13 14:09:51 EST 1995


Dear students,

your questions are realy good and I like to answer your second question, how the seed know to grow upwards.

Even though the seed itself knows nothing, the young seedling can notice the gravity. In the tip of root and shoot there are special cells which are called statocyts. These cells contain small heavy starch-particles (statoliths) which can press on, under the influence of gravity, pressure-sensitive membranes. This is the signal to produce phytohormones. The phytohormones regulate the development of the other cells. The processing of the signal in roots (positive gravitropism) and shoots (negative gravitropism) are contrary.
When the tip of the shoot is outside of soil then it is oriented by light. For light there is a special molecular antenna in the epidermal cells, the phytochrome. This regulte the synthesis of phytohormones and as a result the growth of the basal cells.

Hopefully other scientists will answer your the other questions.



Joerg Leipner
Institute of Plant Science
ETH Z|rich

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