INVEKTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 593 



materials of plants, has been subjected to rigid investigation by 

 Mikosch,* who has arrived at the following results : — 



1. In all young organs whicli are filled with starch-grains, the 

 chlorophyll-grains are formed by the investiture of the starch-grain 

 with a green or yellow protoplasm. Within this envelope a gradual 

 absorption of the starch goes on, followed, when the starch-grain is 

 compound, by its breaking up into its derivative grains. Grains 

 formed in this way he calls starch-chlorophyll-grains. 



2. The starch-chlorophyll-grains are, as a rule, parietal ; only in 

 the leaf-stalks of the primordial leaves, and in the young axial organs, 

 do they more often originate from a mass of protoplasm which 

 surrounds the central nucleus. 



3. The starch-chloroijhyll-grains are functional when fully de- 

 veloped; they assimilate, and multiply by division. The only ex- 

 ception to this rule occurs in the case of the large grains found 

 in the cotyledons of the pea, in which no vital activity was ever 

 observed. 



4. When no starch is present in the tissue in the form of grains, 

 the chlorophyll-grains are formed in the way already described by 

 Sachs, by the breaking up of the hyaline parietal layer of protoplasm 

 into separate green portions, which eventually become yellow. These 

 grains, formed directly out of the protoj)lasm, without a starch-grain 

 taking any apparent share in the process, Mikosch calls protoplasm- 

 chlorophyll-grains. The differentiation of the protoplasm goes on 

 both in the light and in the dark ; but is more rapid in the former 

 case, and, within certain limits, the more raj)id the greater the intensity 

 of the light. 



Heliotropism of Plants. — Professor Wiesner reprints from the 

 ' Denkschriften der math.-naturw. Classe der kais. Akad. der Wissen.' 

 of Vienna, vol. xxxix., the first part of an important monograph, ' Die 

 heliotropischen Erscheinungen im Pflanzenreiche.' 



After a copious historical sketch of previous investigations, he 

 divides the subject as follows :— In the first chapter the influence of 

 the intensity of light on heliotropism is investigated ; and the author 

 shows that a maximum effect is produced by a certain intensity, on 

 each side of which there is a gradual decrease to zero. In the same 

 manner he found an upper limit of light-intensity for growth in 

 length, and also that the upper limit for heliotropism is above or below 

 that for growth in length. Direct sunlight may completely arrest 

 growth in length. In the second chapter he shows, in opposition to 

 the received theory, that all rays of light, with the exception of yellow, 

 even the dark and ultra-violet (red ?) rays of heat, may produce helio- 

 tropism ; in plants which are but little sensitive to heliotropism, the 

 action of the colours of the spectrum decreases in proportion to their 

 heliotropic power. The hypothesis that the heliotropic power of light 

 is in proportion to its mechanical intensity or thermal power is alto- 

 gether disproved. The third chapter is devoted to the investigation 

 of the connection between heliotropism and gcotropism ; and in the 



* ' SB. Akml. Wiss. Wion,' Ixxviii. (1878) p. 205. 



