332 



SCIENCE- GOSSIP 



of the phloem strand known as the " sieve-tubes " 

 (see fig. 3). Each sieve-tube is an elongated cell, 

 with its end walls thickened and perforated by a 

 number of holes, through which at certain times of 

 the year a direct protoplasmic and structural con- 

 tinuity is established. There is in each element a 

 layer of parietal protoplasm, in which are nume- 

 rous drops of mucilage ; and adjacent to each 

 adult sieve-tube is another elongated cell of smaller 

 calibre, the " companion-cell," which is full of a 

 granular protoplasm and has a well-defined 

 nucleus. The sieve-tubes act chiefly in the dis- 

 tribution of the organic food-material of the plant, 

 the mucilage containing much proteid that gives 

 a brownish coloration on addition of iodine. At 

 certain times in the year the sieve-plates are closed 

 on either side by a thick cushion of callose, the 

 ' ; callus," and it is at these periods that the 

 storing function of these elements comes into 

 play. In the spring these cushions are dissolved, 

 and the organic materials pass on and are further 

 distributed. The downward progress is aided 



chiefly by the pressure of turgescent cells in their 

 immediate vicinity. 



There exist also sieve-plates on the side-walls of 

 each sieve-tube, and by means of these organic 

 material passes into the companion -cell, or else- 

 where, where it is again dissolved, and passes, 

 probably in some other form, by osmosis into the 

 surrounding cortex or the cambium. 



We have now studied some of the chief methods 

 in which the nutrient fluids, the raw and elaborated 

 sap, of a plant are conducted and distributed. 

 During our observations we have seen that many 

 physical forces or processes are brought into play, 

 such as osmosis, evaporation, and capillarity. An 

 exhaustive study of these various methods is re- 

 quired in order to fully understand the true relations 

 between physical forces and vital activity, and it 

 must not be here supposed that the former are the 

 only agents. In fact, the tw-o classes aid one 

 another in the furtherance of all these phenomena. 



53 Ordnance Road, 



St. John's Wood, London, N.W. 



COLOURING OF WATER BY MICRO-ORGANISMS. 



Bv James Burton. 



TT is well known, not alone to microscopists, 

 - 1 - that large or small bodies of water are some-- 

 times coloured by the presence of various organ- 

 isms, either animals or plants, often of microscopic 

 size. Every roadside pond is liable to become of a 

 thick soupy appearance and green colour from the 

 multiplication in it of the very common Euglena. 

 or some other of the unicellular algae, such as 

 JProtococcus. Frequently portions in similar locali- 

 ties appear pink or red. owing to the existence in 

 them of immense numbers of some of the Daphniae 

 or water-fleas. In the two cases now to be de- 

 scribed, the colour, though extremely marked and 

 characteristic, was the result of the presence of less 

 common organisms. 



Early in October the ornamental water in the 

 Botanical Gardens, Regent's Park, appeared of an 

 almost uniform pale green. On close examination 

 this was seen to be due to some minute bodies 

 diffused through the water ; they were not merely 

 floating on the surface, but seemed about equally 

 distributed at all visible depths. Every twig and 

 thread of water-weed, etc., at the margin was 

 covered with what looked to the unassisted eye 

 like tiny green balls, while in the quiet corners 

 and backwaters towards which the breeze was 

 blowing, the same bodies were collected in such 

 quantities as to resemble thick light-green paint. 

 Under the microscope it was found that the tiny 

 balls were of irregular outline, and consisted of 

 small algae in colonies of various sizes, formed of 



more or less spherical groups. These were made 

 up of very numerous individuals, oval or pear- 

 shaped, so minute that the green colour noticeable 

 in the aggregations was not distinguishable in 

 them. The groups were hollow and surrounded 

 by a thin layer of jelly or mucilage. In many 

 cases there seemed to be spines radiating from 

 the individuals, but these have no real existence, 

 and the appearance is probably due to the mucilage 

 composed of the swollen outer cell-walls of the 

 separate members not having yet entirely 

 coalesced. 



The colonies, I think, have no motion within 

 themselves, but. being of nearly the same specific 

 gravity as the water, are very readily moved about 

 by any slight current, such as would be set up by 

 wind, or by the sun shining on the surface and 

 causing a difference of temperature between dif- 

 ferent layers. Owing to the disengagement of gas 

 under the influence of light, there is a tendency in 

 the organisms to rise to the surface, while the 

 gelatinous envelopes make them cling to one 

 another and to any object with which they come in 

 contact. Thus are larger and more noticeable 

 masses formed, which, however, have very little 

 cohesion, and disperse again readily. My some- 

 what doubtful identification of Coelosphaeriwm 

 hatzi/iginnum was confirmed by an authority who 

 kindly took the trouble to examine specimens. A 

 figure is given in Dr. Cooke's " Introduction to 

 Freshwater Algae," and the size of the individual 





