Chapter VI 



— 45 — 



The Plastids 



The chloroplasts in vascular plants: theory of Schimper:- In 

 vascular plants, pteridophytes and phanerogams, as has been said, 

 chlorophyll is not present in embryonic cells and is formed only in 

 organs exposed to light as the cells of the organs differentiate. 

 Chloroplasts are therefore found only in green tissue. The question 

 as to the origin of chloroplasts remained a long time unsolved. The 

 older botanists concluded that these bodies were formed by differ- 

 entiation from the cytoplasm or that they were the result of the 

 transformation of starch grains. Schimper (1881-1885) showed 

 for the first time that the chloroplasts are derived in reality from 

 small colorless bodies, protein in nature, which are found in all 

 the colorless tissues of plants. To these bodies he gave the name 

 leucoplastids. These were later called trophoplasts (Meyer) and 

 leucoleucites (VAN TiEGHEM). We shall call them leucoplasts^. 

 These are very small, rounded, 

 or rod-shaped elements which 

 are found scattered in great 

 numbers in the cytoplasm and 

 which are transmitted from cell 

 to cell by division. All these 

 leucoplasts have the ability to 

 form starch grains, i.e., to con- 

 dense in the form of starch, the 

 hexoses elaborated in green tis- 

 sue during photosynthesis and 

 later transported to colorless 

 tissues (roots, tubercles, etc.). 

 Whenever the hexoses accumu- 

 late in green tissues in too high 

 concentrations, the leucoplasts 

 become amyloplasts. 



One of the classic examples 

 of this formation of starch 

 through the agency of the leuco- 

 plasts is to be found in the root 

 of Phajus grandifolius in which 



Schimper succeeded in following the entire process. In this root, 

 the leucoplasts are indeed rather large, rod- or spindle-shaped, 

 bodies enclosing each along its axis a needle-shaped crystalloid cf 

 protein, the product of its elaboration. The starch grain arises in 

 a peripheral region of the thicker part of the leucoplast. Minute 

 at first, this grain grows and very quickly protrudes beyond the 

 plastid which it no longer covers except on one side. 



Leucoplasts develop in various ways depending on the organs 

 in which they are found. In leaves, they have only to grow larger 



Fig. 14. — Chloroplasts containing 

 protein crystalloids from fruit cells. 1, 

 Maxillaria triangularis. 2, 3, Cerinthe 

 minor. 4, Phajus grandifolius. (After 

 Schimper). 



^Arthur Meyer distinguishes between the autoplasts (chloroplasts) and the trophoplasts, 

 the latter comprising the leucoplasts and the chromoplasts. Van Tieghem replaces the term 

 of plastid or plast by that of leucite and distinguishes the leucoleucites (leucoplasts), the chloro- 

 leucites (chloroplasts), and the chromoleucites (chromoplasts). The term plastid having been 

 used by some biologists to mean cell, we replace it therefore by that of plast (which signifies 

 elaborative body) which seems to us much more justified than that of leucite (white body) 

 and we shall distinguish the leucoplasts, amyloplasts, chloroplasts and chromoplasts. 



