Vitrification and Morphological Variations - Some shoots developed in vitro appear brittle, glassy and water-soaked; this is called vitrification or hyperhydracity.

In many species, vitrification may be represented by symptoms not visible to the naked eye, e.g., poorly developed vascular bundles, abnormal wax quality, abnormal functioning stomata, etc. Vitrification is the consequence of culture conditions, and leads to losses of plantlets.

It may be overcome by the following:

(i) increased agar levels (Ca. 1 %),

(ii) bottom cooling of culture vessels,

(iii) addition of agar hydrolysates, and

(iv) use of growth retardants.

Morphological variants may arise during the multiplication state. Such variants occur, in most cases, at a frequency comparable to that in vivo. But in commercial ventures using meristem cultures, visual selection is practised to eliminate variants and maintain the homogeneity of plantlets produced.

Many commercial enterprises, therefore, prefer to multiply shoots for only four or so cycles from an explant; after this, a fresh batch of cultures is initiated from field-tested plants.

Vitrification is a gradual process during which the materials that melt most easily do so, dissolving and filling in the spaces between the more refractory particles. The melted materials promote further melting, as well as compacting and strengthening the clay body.

It is also during this stage that mullite (aluminum silicate) is formed. These are long, needle-like crystals which acts as binders, knitting and strengthening the clay body even further.

Please see related link for a complete explanation and all the stages!