1879.] THE GARDENER'S PRIMER. 329 



pipes were empty. But that this is not the case will be evident to all who will 

 try this simple experiment; for the water will keep a common level, and, fire 

 away as we will, this state will not be altered, but the water will get heated 

 slowly by conduction in the flow and return, and at an equal rate : but again 

 fill up the pipes, and there is displacement of the rarified body by one of 

 greater density at a slightly lower elevation, renewed by suction in the pipes, 

 and causing circulation. Further, to show as a siphon that a high elevation 

 is no promoter of quicker circulation, take a circular vessel, A, 4 feet deep, 

 2 feet in diameter, and standing 4 feet higher than B, of equal size ; fill A full 

 of water, and then bring your siphon 12 feet high or more from A to B. Let 

 C and D be vessels of an equal size to A and B. Also, stand C 4 feet higher 

 than D. C being full of water, bring your siphon only 2 feet high, the siphon 

 reaching the bottom of each vessel. Let the water run in both siphons at the 

 same time, and 2 feet will be drawn from A and C to B and D in the same 

 time when the water in the vessels will have found a common level, and the 

 siphon will cease to act, showing that no circulation can take place until the 

 water in the boiler is made lighter and is displaced by the weightier column, 

 and that it is not necessary to have a continous rise or a high elevation to 

 bring about this result. Albion. 



THE GARDENERS PRIMER. 



NO. IV. 



It is in the mode of arrangement of the vascular tissue that the 

 difference between the stems of monocotyledonous and dicotyledonous 

 plants depends, and which has given rise to the use of the words 

 " endogenous," as applicable to the stem-growth of monocotyledonous 

 plants, and " exogenous," as applicable to the stem-growth of dicoty- 

 ledonous plants. In the endogenous structure they are disposed 

 throughout the cellular tissue of the stem, without any arrangement 

 of pith, medullary rays, or bark, though of course there is the epidermis. 

 The structure of exogenous stems requires a longer description, and 

 then it will be only an imperfect one. Commencing at the centre of 

 the stem axis is the pith or medulla, formed of cellular tissue ; then 

 surrounding it is the medullary sheath, formed of spiral vessels ; from 

 the pith towards the bark, periodically continued by the new growth, 

 are flattened cells, called pith rays or medullary rays, — they apparently 

 keep up the connection with the pith and the new growth, which 

 would otherwise be shut off by the heart-wood, and also strengthen 

 the stem fabric ; then come layers of cellular tissue, called ligneous 

 tissue or woody fibre (the older portion of cellular tissue hardened by 

 deposit, and through which little or no sap can flow), called heart- 

 wood or duramen, the use of which to the stem is to give it mechani- 

 cal strength ; next come the younger layers of cellular tissue c -ltside 

 the duramen, called the sap-wood or alburnum. It is through the 



