Guilliermond - Atkinson — 52 — Cytoplasm 



pierced by colorless pores, encircling a central vacuole which, when 

 visible, contains starch. The plastid is coated by an adsorbed layer 

 of cytoplasm. ZiRKLE thinks starch is formed in the vacuole of 

 these chloroplasts. Living chloroplasts, however, are very delicate 

 organelles and one runs the risk of some alteration taking place 

 during observation, so that it is wise to be very cautious in the 

 acceptance of this structure which has never received any con- 

 firmation. 



More interesting is the later work of numerous authors whose 

 results are in agreement and will be briefly analyzed here. In the 

 first place the works of Menke, Kuster, Heitz, and Friedl Weber 

 have shown that chloroplasts are clearly birefringent. This is 

 particularly easy to demonstrate in the chloroplasts of the Con- 

 jugatae {Mougeotia, Zygnema, Closterium, Spirogyra) . In polar- 

 ized light the chloroplasts are luminous and show a green polariza- 

 tion color. Menke found that in the chloroplasts of the Diatoma- 

 ceae, the polarization colors vary from reddish brown to red. Ac- 

 cording to Weber, reddish brown to red polarization colors are not 

 peculiar to the chloroplasts of the algae (diatoms, Spirogyra, Zyg- 

 nema, Vaucheria) but are also found, together with green polariza- 

 tion colors, in the chloroplasts of higher plants (Polygonatum 

 officinale, Bellis perennis, Elodea canadensis). Weber finds that 

 the perception of these colors depends upon the intensity of the 

 light in which the plants are observed. Furthermore, the investi- 

 gations of Menke, Doutreligne, Wakker, Wieler, Heitz, Weber, 

 Hubert, Geitler, Deschendorfer, Beauverie, Weier and Strug- 

 GER seem to confirm in chloroplasts the existence of a structure, 

 described by Schimper and Meyer fifty years ago, according to 

 which chlorophyll is fixed on grana suspended in a colorless stromal 

 These grana according to Wieler are enclosed within a peripheral 

 layer of the stroma. Heitz thinks the grana are small bodies, 

 measuring from 0.3-1.7/i. in diameter, and appearing flat and 

 discoid in shape, which explains the striated structure often visible 

 in chloroplasts when observed in certain positions. Doutreligne 

 describes them as rods or granules which, according to the condi- 

 tions present, may assemble like strings of beads, may separate, 

 or may become confluent. BEAUVERIE confirms these data without 

 admitting, however, that the structure is general, for he thinks 

 that in certain plants the chloroplasts may be homogeneous. 



The chemical nature of the grana is still unknown. Wieler 

 concludes that they are formed of an essential oil in which the 

 chlorophyll is dissolved. Heitz considers them simply as grains 



'Some even older observations would seem to support this view. Chodat, for instance, in 

 the pseudobulb of Calanthe Sieboldi, described chloroplasts which are round at first but capable 

 of changing to a dumb-bell shape by elongation and stretching at their middle region. Now 

 the two swellings remain filled with chlorophyll while the slender part connecting them becomes 

 colorless. In the mesophyll of leaves and bracts of Iris germanica as well, we have seen large 

 chloroplasts showing at one extremity, or both, a sort of slender, colorless appendage. 

 Embebger observed that in the bulb scales of Lilium eandidum the chloroplasts are surrounded 

 by a colorless layer which seems to indicate that chlorophyll, like the starch grain, has been 

 laid down within the plastid. 



