i'HE A\(;i()SPERMAK : LKAN KS ,^73 



only one vegetative season and are then shed in a senescent condition, although 

 still alive. This only takes into account their life as adult leaves, but in many 

 instances they are formed in the bud during the early part of the preceding 

 season and remain enclosed in the bud in an immature state throughout 

 the following winter before developing to maturity, so that they may properly 

 be said to live for two seasons. Even the so-called evergreen leaves have a 

 relatively short lifetime, indeed it may be said that the difference between 

 a deciduous plant and an evergreen is only that the latter never sheds all its 

 leaves together. In many evergreens among the Dicotyledons the leaves 

 last no more than one year, while those which are longest lived endure for 

 not more than five years. Among some of the slow growing Monocotyledons, 

 such as Agave, this age is probably exceeded, but details are lacking. 



Leaf Differentiation. 



The development of leaf rudiments from their meristematic primordia 

 at the growing point of the stem is by no means simple and we are still far 

 from a full understanding of the process. Yet the importance of the leaf 

 rudiments, not only morphologically but physiologically, and their dominant 

 part in the development of the shoot can be understood when we realize 

 that these minute meristematic rudiments produce the auxins which both 

 stimulate and inhibit growth in all parts of the shoot. 



We have already pointed out that leaves are invariably lateral outgrowths ; 

 to this we may now add that they are invariably outgrowths from meristem 

 tissue and are never adventitious, though, in some exceptional cases, the 

 meristem from which they come may not be that of the stem apex. In some 

 monocotvledonous seedlings, as in the sporelings of some Ferns, the formation 

 of the first few leaves precedes the organization of an apical growing point, 

 while the cotyledons are never, in any case, the product of a stem apex. 

 Indeed the cotyledon of Monocotyledons such as Iris is truly terminal. The 

 anomalous cases of leaf origin in Lenma and in the Podostemaceae have 

 already been mentioned (p. 828). The extent of the apical meristem which is 

 involved in the formation of a leaf primordium appears to be very variable. 

 Among the known cases the majority seem to originate from the tunica, 

 especially where this is two or more layers deep, but in other cases the corpus 

 certainly takes part as well. The earliest part of the rudiment to appear is 

 that which later forms only the apex of the mature leaf, and in some plants, 

 especially some tropical climbers, this first portion of the rudiment grows 

 precociously to form an elongated point or forerunner in which tissue differ- 

 entiation, including venation, may precede the appearance of the rest of the 

 lamina, of which, however, it may later become an indistinguishable part. 



The earliest growth of the rudiment is strictly apical, but this lasts, with 

 few exceptions, only for a short period. In most Monocotyledons apical 

 growth ceases when the rudiment is less than 0-5 mms. long, and the greatest 

 part of leaf growth is therefore intercalary. In Dicotyledons the end of apical 

 growth is usually somewhat later, when the leaf is several millimetres in 

 length. Certain striking exceptions are known in which the apex retains 



