284 DEVELOPMENT OF SIMPLE LEAVES. [BOOK i. 



particular form, are nothing but normal leaves at their birth, 

 the lamina not having as yet been developed, whilst the 

 petiole has taken the form of a lamina. 



No direct researches have as yet been made on the influ- 

 ence which the vascular bundles (nervures) have on the 

 development of the forms of leaves. 



"Development of a Simple Leaf. A rudimentary dicotyle- 

 donous leaf is in shape like a broad or narrow, sessile, entire, 

 crenate or lobed scale, with or without small lobes (stipules) 

 at its base, continuous with the lamina. Whilst the top of the 

 leaf is distant from the axis, and the upper part of the scale 

 is old at the same time the lower parts are still in a nascent 

 state. The first important transformation that takes place in 

 a rudimentary leaf is the development of the blade. 



These blades may be divided into two classes. They are 

 either developed so as to be applied by their whole surface to 

 other parts of the bud, or they are folded outwardly or 

 inwardly at their two halves. (Vernatio duplicativa and 

 replicativa.) In the first case we see the large rim that con- 

 stitutes the lamina, extend its edges more and more above 

 the parts on which it reposes, until it envelopes them ; it at 

 the same time diminishes in thickness, the edges being the 

 thinnest parts. There sometimes remains on its surface a 

 projecting medial line; at other times this line disappears, 

 leaving the lamina of nearly equal thickness throughout, as 

 it is found in most Monocotyledons. When the edges have 

 proceeded so far as completely to envelope the youngest parts 

 of the bud, and to touch one another, they often grow together 

 at their base, and folding several times, roll themselves more 

 or less completely round the bud ; whence it happens that 

 the blade is rolled spirally, the spiral being capable of describ- 

 ing many turns of which the most recent are always the 

 nearest to the axis. 



In Dicotyledons convolute vernation is much more rare, 

 for the base of the rudimentary leaf does not generally occupy 

 the whole of the periphery of the axis. When the young 

 foliaceous lamina of these plants is folded round the axis, it em- 

 braces it, either wholly, (by sometimes several convolutions), 



