r'. 



LEAVES 525 



Mougeotia has platelike plastids which stand on edge in intense light 

 and have a horizontal position in diffuse light. 



The usual view has been that plastids are not actively motile but drift passively 

 in cytoplasmic currents, though there is some evidence that they exhibit amoe- 

 boid movements, reacting chemotactically to carbon dioxid and to other sub- 

 stances unequally distributed in the cell ; in any case it is clear that they change their 

 form as conditions change (figs. 758, 759). Other factors than light cause change 

 of position, notably changes in temperature and in water content. In Dictyota, 

 apostrophe is caused by immersion in water of higher concentration than sea water, 

 while immersion in water of lower concentration results in epistrophe. Possibly the 

 plastid movements commonly referred to changes in light intensity may be due in 

 part, at least, to osmotic changes; at all events, the effects of reduced water supply 

 and of high light intensity seem to be the same. 



It is likely that plastid motility has been too much emphasized. Intermediate posi- 

 tions are much more common than either apostrophe or epistrophe. Sun plants 

 in particular have plastids that are relatively immotile, the position of apostrophe 

 (profile position) being usual in darkness as well as in daylight (fig. 766). The 

 greatest motility is found in water plants and in shade plants ; in sun plants the 

 plastids near the under surface are more likely to change their position than are 

 those in the elongated or palisade cells near the upper surface. 



The synthesis of carbohydrates. The water absorbed by the roots 

 and the carbon dioxid absorbed by the leaves combine in the latter in 

 the presence of sunlight and form elementary carbohydrates, the pro- 

 cess being accompanied by the emission of oxygen. Generally the 

 theory has obtained that the first carbohydrate formed is formaldehyde, 

 and that later through its condensation there develop sugars, which may 

 migrate as such through the plant or which may be transformed into 

 starch by the plastids; in the blue-green algae, glycogen is an early syn- 

 thetic product, and fatty oils are the first visible products in Vaucheria 

 and in diatoms. The recent discovery of formaldehyde in the plastids 

 of numerous leaves tends to confirm this theory, the objection that it 

 is poisonous to plants being met by assuming that it at once becomes 

 condensed into harmless carbohydrates. 



The exact seat of synthesis is not surely known, some investigators holding that 

 it is in the plastid, others that it is in the pigment, and yet others that both are neces- 

 sary. Formerly the chlorophyll was thought to be a screen which absorbs rays dele- 

 terious to plastid activity. A prevalent modern view regards the plastid as the chief 

 synthetic factor, the chlorophyll acting as a sensitizer. Another view is that chloro- 

 phyll is the chief synthetic factor and that the plastid furnishes a convenient seat 

 of activity, particularly because of its power to transform sugar into starch. Many 

 investigators regard enzyms as having an important part in the process. In most 



