ABSTRACTS OF FOUR LECTURES TO STUDENTS. 



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ABSTRACTS OF FOUR LECTURES TO STUDENTS, AT 

 CHISWICK, 1903. 



By Rev. Pkof. G. Henslow, M.A., F.R.H.S., V.M.H. 



I. Arrangement and Stipulation. 



Arrangement. — Leaves are arranged in two ways on a shoot ; either 

 singly at a joint or node, when they are said to be alternate in position ; 

 or else two or more stand at the same node. When this is the case they 

 are opposite, if two, or whorled, if more than two ; and the rule is 

 that each pair, or more than two if whorled, alternate in position with the 

 pair or whorl above and below. 



There is great variety of arrangements among alternate leaves, and 

 they may be represented as follows : 



If any leaf on a shoot be called No. 1, and the next No. 2, and if this 

 stand on the opposite side of the stem, then the third leaf, or No. 3, will be 

 exactly over No. 1. A line drawn on the stem from No. 1, through No. 2, 

 to No. 3, would coil spirally once round the stem. It would form a 

 circle if compressed like a watch-spring, instead of being like one turn of 

 a screw. 



Sometimes the fourth, sixth, or ninth leaf falls over the first. In the 

 last two cases the spiral line makes tivo coils. 



A simple way to represent these different arrangements is to let the 

 number of leaves 2, 3, 5, 8 — that is, from No. 1 to the next leaf exactly 

 over it, but not including it — be denominators of fractions, the numerators 

 being the number of coils on the spiral line. 



Hence the four kinds mentioned would be represented as follows : 



1 1 2 | 

 2> 3> 5' 8 



Each of these denominators is said to make one " cycle " ; the third, fourth, 

 sixth, and ninth leaves over the first, respectively, begin the next cycles. 



It will be at once observed that the sum of any two successive 

 numbers makes the next numerator and denominator respectively ; so 

 that we might continue the series thus : 



5 8 13 Sr n 



The first four are the commonest when leaves are not crowded, but when 

 the "internodes " between them are very short, then the arrangement of 

 the leaves (or what may represent them, as the bracts of the involucres of 

 Composites, or the scales of cones), will be found to be represented by the 

 higher fractions T %, ¥ 8 X , JJ, &c. 



The question may soon be asked, What is the meaning, or use to the 

 plant, of these different arrangements ? The answer is, that the leaves 

 may present as much as possible of their upper surfaces to the light ; so that 

 when they are crowded, it may be not until we get to the fourteenth, twenty- 

 second, or thirty-fifth leaf that each falls almost exactly over the first. 



A plant has the power to change its normal arrangement on the 

 same plant, on its different shoots, according to their position. Thus the 



