Indagini Al Microscopiop Ottico Ed Elettronico Su Apici Vegetativi Quiescenti in Talee DiPsilotum Nudum(L.) Beauv

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Abstract
Researches at the optical and electron microscopes on quiescent vegetative apices of « Psilotum nudum » (L.) Beauv. cuttings. Small aerial branches of Psilotum nudum have been planted in sand and kept in a very humid hothouse. None of them has rooted and no noticeable growth occurred. However the cuttings have remained vital during three years and one month until the moment that is when the last one was utilized for our researches. These cuttings and some other ones planted at a later date were utilized for a study of the quiescent vegetative apices at the optical and electron microscope. Material was collected 15 days, one month, three months, one year, two years, two and a half year, and three years and one month after planting. We have noticed a progressive thickening of the cell walls, reabsorption of the plasmodesmata and a separation of the individual cells which were no longer kept together by the middle lamella: all these changes bring about a kind of encystment of the apical cells. We have also observed a progressive accumulation of starch grains in the chloroplasts until the cells appeared absolutely full of starch. In the two years, and the three years and one month old cuttings a gradual decrease of the starch content occurred, especially in the cells nearest to the stem tip. We are led to think that the exceptional capacity for survival which distinguishes Psilotum nudum could be explained to a large extent by the attainment of the « encysted » condition of its apical cells together with their capacity of storing a great quantity of starch.