DEVELOPMENT OP LEAVES. 65 



constitutional or accidental, then they remain fixed in the 

 state in which the arrest occurred. Owing to this cause, we find 

 them in the form of points, scales, straps, or perfect organs on 

 the same plant. 



The Mstory of their development has heen explained by M. TrScul 

 better than by any other writer. The foUowing is the substance of 

 his remarks : — The stem terminates in a very deKcate cellular tumour 

 (growing poiat) from the sides of which the leaves are developed. 

 These first present themselves in the shape of stiU. smaller tumours, 

 alternate, opposite, or verticillate. When opposite or vertioUlate leaves 

 are to be united at the base, a circular elevation precedes them on 

 the axis ; when they are not confluent the tumours are isolated ; lastly, 

 when alternate leaves are sheathed, the sheath either takes its rise 

 from a circular eminence round the stem, or else the rudimentary 

 tumour which first shows itself, enlarges, and finally embraces the 

 stem. Leaves are developed after foxir principal types, the centrifugal 

 (from below upwards), the centripetal (from above downwards), the 

 mixed and ^e parallel. In the CBNTEinjGAL formation all the parts are 

 formed from below upwards, the leaf is pinnate, and furnished with 

 stipules, the petiole (raohis) first makes its appearance ; on its sides 

 come the stipules, then the lower pair of leaflets, then the second pair, 

 then the third, fourth, and so on. If the leaf is supra-decompound the 

 primary tumoxu or raohis in growing throws out secondary petioles, and 

 these latter tertiary ones, &o., according to the composition of the leaf 

 at the extremity of which the leaflets form. The development of simple 

 leaves may be explained by that of the Lime-tree. This leaf commences 

 with a rudimentary tumour at the apex of the stem. This tiunour 

 lengthens and enlarges, leaving at its base a contraction which repre- 

 sents the petiole. The blade, at first entire, is soon divided from side 

 to side by a sinus. The lower lobe is the first secondary nervure ; the 

 upper part is subdivided in the same manner five or six times, in order 

 to form as many nervures of the same sort. About the time that the 

 third or fourth upper lobe makes its appearance, the lower one, which was 

 formed first, having also extended, becomes sinuous at its edges. These 

 sinuosities are the indications of the origin of five or six ramifications 

 of the lower nervure. At this period the leaf is furnished with as 

 many toothings as there are nervures. But in a short time fresh 

 toothings appear between those first formed, these correspond with the 

 development of as many secondary nervures. The nervures which 

 unite transversely with the adjoining nerves are produced at the same 

 time. The hairs which cover the under surface of the leaf are also 

 formed from below upwards. Thus the various kinds of nervures in the 

 leaf of a Lime-tree develope like the different sorts of shoots in the tree 

 that bears them. To leaves developed centkipetallt belong those of 



