LEAF 367 



at night, or movements during heat or dryness (cf. Movements). The 

 fall of a 1. is often effected by an absciss-layer, or stratum of cork 

 which separates it from the stem, and then splits, leaving a leaf-scar 

 upon the stem. L. with such a layer are called articulate, without, 

 non-articulate. 



Under other than mesophytic conditions such structure as just 

 described shows many modifications; cf. Climbing Plants, Insecti- 

 vorous Plants, Parasites, Saprophytes, Water Plants, Xerophytes. 

 It is also modified for Storage (g.v-) of reserves, or for Vegetative 

 Reproduction (q-v.). 



In external form 1. show very great variety, and the grouping of 

 gen. into sp. is largely determined by this, so that it is necessary to 

 understand the use of the technical terms employed in such work, 

 before one can use a flora with any advantage. An outline of such 

 terms and their use follows. 



Descriptive Terms. The student should practise describing leafy 

 shoots until expert in handling terminology, but there is no need to 

 commit the terms to memory. At first he should describe in detail 

 in the order given below, but afterwards try to render his descriptions 

 short and pithy without sacrifice of essentials; this can only be well 

 done by comparison with related forms to see what points are common 

 to all. 



L. as to phyllotaxy (q.v-} or arrangement may be radical, or on 

 the subaerial stem (cauline) ; whorled (i'erticillate), opposite (and then 

 decussate if each pair is _L the next, connate if the two are con- 

 crescent as in Lonicera, anisophyllous if unequal in size or shape), or 

 alternate (the phyllotaxy fraction may be given, or the number of 

 ranks described by the terms di-, tri-stichous, &c.). With regard to 

 insertion or mode of union with the stem the 1. may be petiolate or 

 sessile (i.e. with or without stalk respectively ; the petiole is descr. 

 like a stem), auricled (with two lobes of the blade overlapping the 

 stem), amplexicaul (the lobes clasping the stem), sheathing (as in 

 Grasses, the leaf-base forming a tube round the stem), perfoliate (the 

 leaf united round the stem, as in Bupleurum), Recurrent (continued 

 by a wing on the stem, as in thistles), &c. It may bear a ligitle or 

 scale at the upper end of the leaf-base or sheath, as in Grasses. It 

 may be stipulate or exstiptilate (with or without slip, respectively); 

 the shape, &c. of the slip, is described as if they were 1. and they 

 may be free or adnate (F in fig., concrescent with the leaf- base or 

 petiole, as in rose), united to other slip., inter- or intra-petiolar, 

 branched, &c. (see Rubiaceae), ochreate (sheathing, as in Polygo- 

 naceae), or modified in various ways. The venation (arrangement of 

 the veins) may be pinnate or palmate ; in the former case there is a 

 midrib with lat. veins branching from it, in the latter several equal 

 veins spread out in the 1. like the ribs of a fan, from one point. The 

 further ramification of the veins is descr. by net-veined (irreg. mesh- 

 work, as in most Dicots.), parallel-veined (meshes more or less rect- 

 angular, as in most Monocots.), fork-veined (veins forking into two, 

 as in Ferns). 



L. are divided into simple and compound, as the stalk bears one 

 or several separate leaflets. In the latter case the leaflet is desc. as if 



