ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 485 



each oospbere receives only one of the two. This is the law of " split- 

 ting " (disjonction, Spaltung~). 



Hybridisation in Citrus.* — Mr. H. J. Webber points out that in 

 polyembryonal seeds of Citrus which are the result of hybridisation, only 

 one of the embryos shows any trace of the characters of the male parent, 

 while all the others resemble the female parent. Probably tbe true 

 bybrid is derived from the fertilised oospbere, all the others from adven- 

 titious embryos produced in the nucellar tissue. 



(2) Nutrition and Growth (including 1 Germination, 

 and Movements of Fluids). 



Physiological Action of Mineral Nutrient Substances.f — HerrO. 

 Loew has investigated the part played by various mineral substances in 

 the nutrition of the plant. 



Phosphoric acid is used up in the formation of lecithin and of nucleo- 

 proteids, e.g. chromatin and plastin. Salts of iron are utilised in the 

 formation of chlorophyll and of haemoglobin. Iron is universally present 

 in Fungi. Potassium salts are absolutely necessary for the thrift of the 

 plant ; they cannot be replaced by corresponding salts of sodium or 

 lithium. Calcium and magnesium occur in all parts of plants ; the 

 leaves usually contain more calcium, the seeds more magnesium, than 

 the other parts. Calciuru-proteinaceous substances appear to be neces- 

 sary to the constitution of the organised substances from which the 

 nucleus and the chlorophyll grains are formed ; the calcium cannot be 

 replaced by other bases. Calcium salts promote the growth of the root- 

 hairs. They are not, however, necessary to the life of Fungi, Bacteria, 

 or the lower Algse. In the absence of calcium, magnesium is poisonous 

 to the higher plants. It is readily dissociated, and hence assists assimi- 

 lation, especially of the phosphoric acids, and is therefore of great 

 service in the formation of the nucleo-proteids. Magnesium salts are 

 necessary for the thrift of Fungi. 



Effects of Diminished Pressure on the Growth of Plants.} — Herr 

 F. Schaible confirms the statement of Wieler and Jaccard that the 

 growth of plants, and especially of the leaves, is promoted by a diminu- 

 tion of the atmospheric pressure, while germination is retarded. The 

 latter phenomenon appears to be the result of a diminution of the 

 amount of oxygen in the atmosphere, the former directly of a diminu- 

 tion in the pressure. 



Relation of Arctic Plants to Light. § — Prof. J. Wiesner states that 

 in the Arctic region the amount of light which reaches the plant amounts 

 to very nearly the maximum possible. The leaves are deprived of very 

 little by tbe shade either of the plant itself, or that of other plants ; but 

 it does suffer to some extent from the configuration of the ground. The 

 necessity of a very large amount of light for Alpine plants is a result Of 



* Bot. Gazette, xxix. (1900) p. 141. 



t U.S. Dept. Agriculture; Div. Veg. Phys. and Pathol., Bull. No. 18, 1899, 

 60 pp. 



% Beitr. z. wiss. Bot. (Fiinfstiick), iv. (1900) pp. 93-148 (8 pis. and 3 figs.). See 

 Bot. Centralbl., lxxxii. (1900) p. 52. 



§ S.B. k. Akad. Wiss. Wien, May 3rd, 1900. See Bot. Centralbl., lxxxii. (1900) 

 p. 316. 



