1 6 A TEXTBOOK OF THEORETICAL BOTANY 



in through the cytoplasm. The entry of water creates a positive pressure in 

 the vacuole, which is called the turgor pressure, by means of which the 

 cell as a whole is kept extended. It is interesting to note that the concentra- 

 tions of some inorganic salts in the vacuole may be different from that outside, 

 showing that the function of the protoplasm in absorption is by no means 

 merely passive. 



The term mitochondria is applied to granular elements found in the 

 cytoplasm, some of which are relatively large or filamentous, and readily 

 recognizable even in living cells. Granules of all sizes occur, however, and 

 it is not certain where the limit should be drawn, or whether all particles 

 down to visible limits should be included under this name. There is in 

 cytoplasm a certain class of granules, or short rods with certain chemical 

 properties in common, which may be regarded as true mitochondria. They 

 disintegrate in highly acid or alkaline solutions, and consist of a mixture or 

 compound of phospho-lipins and nucleo-proteins. The nucleic acid in- 

 volved here is, however, not the same as that in the nucleus itself, as described 

 later, but is ribo-nucleic acid, a cytoplasmic material, which may also occur 

 free in the form of granules, named originally volutin grains. 



The universal occurrence of mitochondria suggests that they have some 

 metabolic function, but although theories have been plentiful, facts are scarce 

 and no definite statement can yet be made on the subject. 



The plastids are persistent cell organs, which appear to arise from minute 

 granules present in the youngest cells and ultimately derived from the 

 oosphere. These pro-plastids have often been confused with mitochondria, 

 and whether they are distinct structural elements is still uncertain. The 

 mature chloroplast has been the subject of intensive research on account 

 of its importance in photosynthesis. The variability of its form in the Algae 

 is as striking as is its constancy of character in all the higher plants. This 

 constant form is roughly that of a disc, like a watch, with convex faces, which 

 seems to have proved so efficient that it has remained unchanged through 

 vast periods of evolution. Fully grown chloroplasts multiply in the cells by 

 a process of division. The plastid elongates and a median constriction or 

 waist appears, which narrows until the two halves are completely separated. 

 The plastid is protoplasmic in nature, though of gel consistency, and there 

 is evidence that it has a semipermeable surface membrane. The protoplasmic 

 body or stroma is colourless, but in chloroplasts it includes numerous small 

 bodies, called grana, composed of protein and lipoid, which contain the 

 chlorophyll. Where these are large and not too closely packed, as in many 

 Mosses, they may be seen as green spots, giving the plastid a mottled 

 appearance. 



In the Algae the chloroplasts may contain a protein body called the 

 pyrenoid, around which the starch grains are formed during photo- 

 synthesis. 



Leucoplasts are similar in development and structure but lack chlorophyll. 

 Some at least, however, form chlorophyll when they are exposed to light, as 

 in potato tubers. 



