ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 985 



the second. In all, the sieve-plates are covered by a very thin layer of 

 callus, which is either completely covered with mucilage, or only at the 

 margins of the sieve-pores on both sides of the plate. 



In the development of all three types, drops of mucilage are first formed 

 in the parietal protoplasm. In the Cucurbitaceae these are soon again 

 absorbed ; in other cases they remain ; in the third type the starch-grains- 

 are formed at the same time. The author agrees with Russow that the 

 callus is separated from the contents of the sieve-tube and not from the 

 cellulose-plate. In Cucurbitaceae the sieve-plate is slightly pitted before 

 the formation of the callus, but the pitting is in all cases a secondary 

 process after the tubes have emerged from the condition of cambium-cells. 



The obliteration or cessation of the functions of the sieve-tubes, com- 

 mences with changes in both the contents and the sieve-plates, which vary 

 in different plants. They finally become completely empty ; when they 

 contain starch-grains, these are the last to disappear. The pores also be- 

 come completely closed. 



Dr. Fischer afl&rms that the sieve-tubes are in connection Avith one 

 another and with the conducting cells, but not with the cambiform. 



Effect of Stimulation on Turgescent Vegetable Tissues.* — Miss A. 

 Bateson and Prof. F. Darwin have tried a series of experiments on the effects 

 of water and other reagents on the increase in length of the turgescent pith 

 of a growing shoot when freed from its surrounding tissues. The plants 

 experimented on were Helianthus tuberosus and S. annmts. The increase in 

 length was measured by means of an auxanometer-lever. One end of the 

 pith was attached to the bottom of a narrow glass jar, the upper end being 

 connected, by means of a thread of plaited silk, with the short arm of the 

 lever. The following is a summary of the chief results. 



Turgescent pith placed in water increases in length, at first slowly, then 

 more quickly ; and then again the rate of increase becomes more slow.. 

 The rate of increase in length increases as the temperature of the water 

 rises, reaches an optimum, and suddenly falls as a temperature sufficient to 

 cause flaccidity is approached. The following reagents cause distinct acce- 

 leration: — Alcohol, ether, ammonia, hydrocyanic acid. The first three 

 produce a very temporary effect, whereas prussic acid has a prolonged action. 

 The following reagents produce retardation : — Acetic acid, hydrochloric 

 acid, and probably nitric acid. Dilute solutions of quinine chloride and; 

 of carbolic acid produce a remarkably rapid shortening of the pith. 



Formation of Tyloses in the interior of Secretory Canals.f — Mdlle. A,. 

 Leblois states that she has lately made a series of researches on the origin 

 and development of secretory canals. In the course of the investigation 

 which was made on the branches of Brucea ferruginea, cells projecting; 

 into the interior of the secretory canals were observed. These cells were 

 sometimes in the form of a hair or papilla, but more often they were club- 

 shaped, and were formed by the projection of the cells at the border of the 

 eanal. Afterwards these cells were observed to divide by transverse septa. 

 In the older branches, on account of their number, these cells somewhat 

 filled up the canal ; they then took on the appearance of tyloses. 



The author concludes by stating that two types of tylosis might be: 

 distinguished : firstly, those that occur in the old vessels and which were 

 described in 1845, and, secondly, those shown to occur in old secretory- 

 canals. 



* Journ. Linn. Soc. Lond.— Bot., xxiv. (1887) pp. 1-27 (5 figs.). 

 t Bull. Soc. Bot. France, xxxiv. (1887) pp. 185-6. 



