56 



KNOWLEDGE 



[Maboh 1, 1898. 



defined features in its life history which mark a great step 

 in the evolution of plant life. Firstly, there is the 

 development of the trichogyne. Secondly, there is the 

 influence of the process of fertilization on cells adjacent to 

 the egg-cell, resulting in the formation of a fruit. Thirdly, 

 there is the all- important phenomenon of the division of 

 the carpospore into a group of cells which do not imme- 

 diately reproduce the parent plant. 



We have here, in fact, a very early indication of that o/icr- 

 nation of >i, nriKtinns which has played so important a part 

 in the story of plant life, and a study of which has given us 

 the clue to the relationship between the lower and higher 

 members of the vegetable kingdom. It is here that we 

 see clearly for the first time, in the upward succession of 

 plant types, the distinction between an oiijihyte or egg- 

 bearing generation and a sporojihyti or spore-bearing gene- 

 ration, arising from it and in turn reproducing it again. 



Some of the consequences of the increasing differentia- 

 tion of these alternating stages and the speciahzation of 

 their accessory tissues will be pointed out in later studies. 



CLOUD BELTS. 



By Wm. Shackleton, f.r.a.s. 



ANYONE who has made a voyage beyond the 

 Equator will, no doubt, retain a good recollection 

 of a day or so of disagreeable, oppressive, damp 

 weather, when moisture seemed to be exuding 

 from all sides, just as if one had come out of a 

 dense Scotch mist, and everything was coated with a thick 

 film of moisture which trickled down in great beads. 

 This journey through the watery-laden atmosphere and 

 almost constant rain, is really a passage through the 

 equatorial cloud belt which girdles the planet on which we 

 happen to be located ; and although we may admire Jupiter 

 with his many cloud belts as seen through a telescope, yet 

 we feel thankful for the invention of steamships which 

 enable us to leave behind as quickly as possible the most 

 marked cloud-belt appended to our earth, rather than be 

 becalmed in these " doldrums " where ships have been 

 known to drift listlessly about for whole weeks. 



A graphic description of the kind of weather which is 

 usually experienced under the cloud ring of the equatorial 

 calm belt is found in the journal of Commodore Sinclair, 

 kept on board the U.S. frigate Congress during a cruise to 

 South America in 1817-18. He crossed it in the month 

 of January, 1818, between the parallel of 4- N. and the 

 Equator. He says :— " This is certainly one of the most 

 unpleasant regions on our globe. A dense, close atmo- 

 sphere except for a few hours after a thunderstorm, during 

 which time torrents of rain faU, when the air becomes a 

 little refreshed ; but a hot glowing sun soon heats it again, 

 and but for your awnings and a little air put in circulation 

 by a continual flapping of the ship's sails it would almost 

 be insufferable. No person who has not crossed the region 

 can form an adequate idea of its unpleasant effects. 

 Except when in actual danger of shipwreck, I never spent 

 twelve more disagreeable days than in these calm lati- 

 tudes." 



The general appearance of the sky in this " rainy sea," 

 as it has been called, is a steamy haze — sometimes growing 

 into uniform gloom, with or without heavy rain, at other 

 times gathering into small ill-defined patches of soft 

 cumulus. After dark there is always a great development 

 of sheet lightning till about two in the morning. 



The Plate shows the appearance at the edge of the cloud 

 belt on the confines of the south-east trade wind, and is 

 reproduced by the kind permission of Sir J. Benjamin 



Stone, ]\I.P., from a photograph taken by him in 1894, on 

 his way to South Africa. 



Besides this equatorial cloud belt, however, there are 

 two other rings encircling the earth, where rain falls 

 perhaps more incessantly even than in the equatorial 

 belt itself, though by no means in such large quantities. 

 These latter belts occur near latitude GO in both 

 hemispheres ; and perhaps more of us have passed through 

 these than that of the equatorial belt, especially the 

 one crossing the Shetlands and South Norway about 

 Bergen, where it rains nearly every day throughout the 

 year, and which place tourists speak of as especially 

 relaxing, thus experiencing some of the effects described 

 by Commodore Sinclair. 



It is not necessary to go into detail as to the actual 

 cause of these cloud belts — that is a matter for text-books ; 

 sufficient it is to say that in the case of the equatorial belt, 

 the north-east and south-east trade winds flowing into the 

 equatorial regions to supply the up-draught caused by the 

 intense heating of the atmosphere surrounding the Equator, 

 pass over zones of about twenty degrees in width, from 

 which all, or nearly aU, the vapour of evaporation is carried 

 into the comparatively narrow zone of the equatorial calm 

 belt before it ascends to higher and therefore colder levels. 

 In these upper reaches condensation takes place, thereby 

 producing a constant canopy of dense cloud which forms 

 a nearly continuous cloud girdle. The equatorial calm 

 belt, therefore, is also a cloud and rain belt. 



It has been estimated that the daily amount of evapora- 

 tion on the ocean within the tropics is about a quarter of 

 an inch per day. If, then, all this amount of vapour over 

 zones, say, one thousand miles in width on each side, is 

 carried into the calm belt, say three hundred mUes in 

 width, and is there precipitated as rain, the daily rainfall 

 would be 1-G7 inches; and consequently if this belt 

 were to remain stationary, we should have an annual 

 rainfall of about sixty feet for the average of the width. 

 But since the cloud and rain belt oscillates through a 

 range generally more than twice as great as its width, 

 this amount of rain is distributed in the course of the 

 year over a zone more than three times as wide, and hence 

 in general less than one-third of this amount falls in any 

 one place during the year ; aj., at Maranhao at the mouth 

 of the Amazon, and on the border of the cloud zone, the 

 rainfaU is two hundred and seventy inches per year, and 

 is even greater at several places, but this is chiefly due to 

 local influences. 



From certain causes the rain and cloud belt, as it 

 exists at any given time, is mostly wider than the belt of 

 calms, but of course neither have very definite hmits ; these, 

 however, are much better defined over the great oceans, 

 where the trade winds blow much more steadily than on 

 the continents, where regularity is very much interfered 

 with by the various abnormal disturbances of uneven 

 surfaces and mountain ranges, and likewise by the 

 monsoons of the Indian and other oceans. The rain 

 and cloud belt is, however, clearly traceable across the 

 whole of Africa, wherever observations have been made, 

 as also across the American isthmus ; but it has greater 

 width and its Hmits are not so well defined. These cloud 

 zones, on which large amounts of rain fall, are traced out 

 naturally for us on the surface of the globe, and it has 

 been truly said that these regions are the "reservoirs of 

 the great rivers"; e.ij., those originating from the equatorial 

 cloud belt being the Amazon, Orinoco, Niger, Nile, and 

 Congo, whilst the Yenesei, Obi, Mackenzie, and St. Lawrence 

 largely derive their supplies from the minor belt in the 

 northern hemisphere. 



From certain causes which can be explained, the mean 



