428 Mr Gustav M ann's 



with water, which causes the bags to expand and to fomi 

 bladder-like structures, to which the threads are attached. 

 In S. nitidci the two bags are so dense that they won't 

 swell up, and they seem to have fused completely to 

 form only one circular bag. Fig. 5 represents a portion 

 of the bag surrounding the nucleus with threads going 

 to the pyrenoids, dissected out from a cell of Spirogyra 

 nitida. 



There seems to be considerable diflFerence of opinion about 

 the starch-centre or protein crystal, contained in the cup- 

 like expansion of the threads. A. Meyer, in Bot Zeitung, 

 1883, No. 30, describes the protein crystals as bodies with 

 angular outline ; Strassburger describes them similarly in 

 his Practical Botany; Berthold denies the angular out- 

 line, and also the statement made by Schmitz that the 

 protein crystal consists of a substance identical with the 

 chromatin of the nucleus; A. Meyer and Schimper also 

 deny the first-mentioned identity ; Zacharias considers the 

 nucleoli and pyrenoids to be similar, since both consist of 

 digestible albuminoids. I myself hold that the protein 

 crystals are either globular or angular, according to the 

 amount of starch stored up in the pyrenoids. If a pyrenoid 

 be examined in which there is no starch we see a central 

 rounded body, the protein crystal surrounded by a pale 

 ring, the cup-like expansion of the thread. As starch is 

 being formed it is laid down outside the protein crystal 

 round definite centres, and therefore we see, if a small 

 quantity of starch is present in the pyrenoid, minute 

 granules lying close to the protein crystal. As more and 

 more starch is deposited, these small granules gradually 

 increase in size, and assume a bi-convex shape ; and whenever 

 they have increased so much in size as to touch one another, 

 the protein crystal assumes an angular shape. If assimila- 

 tion has been going on very vigorously, so much starch 

 may be deposited that it is no longer possible to make out 

 the different starch granules, and they will appear to have 

 fused to form a swollen ring-like mass round the protein 

 crystal ; and the latter will have, on focussing, a circular 

 outline, as is best seen in 8. nitida, if threads be examined 

 towards the evening of a warm, sunny summer day. For 

 explanation of the figures see page 431. 



