mVERTEBEATA, CBYPTOGAMIA, MICROSCOPY^ ETC. 825 



the cell-wall is much thinner, not varying greatly in thickness from 

 the sieve-plate itself, the transition into the sieve-plate is more 

 gradual, the boundaries of the latter being less striking to the eye, 

 and the form of the sieve-plate is altered, being divided by stri83 of 

 the cell-wall into smaller plates more or less separated from one 

 another, as in Ginrjko, Gnetum, Ephedra, and Cycas. Mature sieve- 

 plates contain no jn-otoplasm in their interior, and are not subject to 

 any changes from the season. Their pores are always uncovered, and, 

 as in angiosperms, comiiletely perforated. 



The sieve-i^lates arise from the membrane of the bordered pits 

 which are in the cambial cells on their radial and terminal walls. 

 The membrane of these bordered pits swells up, is considerably 

 altered in structure and in chemical composition, and finally forms 

 on both sides a thick callus, within which is the sieve-plate, which 

 is exposed by the absorption of the callus. 



Since the protoplasm disapj)ears from the sieve-tubes immediately 

 after they are set free, nothing can at present be satisfactorily deter- 

 mined respecting their physiological purpose and the period of their 

 activity. 



The sieve-tubes of gymnosperms are therefore homologous to 

 those of angiosperms, but differ from the latter, both in the mode 

 of development of their sieve-plates, in the absence of protoplasm 

 from them when mature, and also in the constancy in form of the 

 sieve-plates at all periods of the year. 



Chemical Composition of Chlorophyll.* — E. Sacchse, of Leipzig, 

 publishes some fresh results of phyto-chcmical investigations on the 

 composition of chlorophyll. By means of a peculiar method he has 

 succeeded in separating both the green and the yellow pigment from 

 the beuziu-extracts of leaves, in Allium ursinum and Primula elatior, 

 in a pure state, though perhaps not altogether unchanged from the 

 chlorophyll in a functional condition. 



Contrary to expectation the green pigment was found not to be 

 homogeneous, but capable of separation into five distinct chemical 

 substances, resembling one another closely in optical properties, but 

 varying in quantitative composition. The proportion of carbon varies 

 between about 66 and 72 per cent., that of nitrogen between about 

 3 and 5 • 5 per cent. 



Similar results were obtained with the yellow pigment. In this 

 also were found at least four distinct chemical substances, varying in 

 colour from yellow to reddish brown, of a similar fatty nature, and 

 similar spectroscopic proi)crties, but varying in chemical composition. 

 They are all non-nitrogenous, while the proportion of carbon varies 

 between about 66 and 71 per cent. Each of the green pigments has 

 a corresponding yellow "pigment with the same carbon-percentage, but 

 destitute of nitrogen. 



In addition to the green and yellow pigments there was found a 

 remarkable substance, agreeing nearly with starch in its carbon- 



* ' Phytoclicm. UnUr.siicli. iRraiisj^. v. R. Sacchse,' i. (ISSO) p. ]. iSco ' Bot. 

 Ccntralbl.,' i. (IbSO) p. 5J'J. 



