|MadSci Network: Cell Biology|
Vinh, Several years back I answered a similar question on this same network but, it was only dealing with the epidermal cells of onions. You can see this answer at: ht tp://www.madsci.org/posts/archives/oct2001/1002637439.Cb.r.html Light is needed for the development of chloroplasts. There are many types of plastids found in plant cells but, not all plant cells have all the different types of plastids. Some plastids can contain pigments other than chlorophyll and are called chromoplasts and plastids that store starch are amyloplasts. It is thought that all plastids arise from a pro- plastids. Since onion bulbs are below ground they do not get the light necessary for chloroplast development. There are lots of web sites available that have information on this subject. Try a google search using the words "chloroplasts and onions" or "onion chloroplasts". If you are interested in learning more about cells, onions are a good experimental organism. Onion root tips are great to study mitosis and meiosis and the thin membranous epidermis between the onion rings are perfect for observing plasmolysis. Good luck with your studies.
Try the links in the MadSci Library for more information on Cell Biology.