i6 2 DIFCEFEREN OF ORGANS AT DEVELOPMENT STAGES 



the plant has given rise frequently to discussion because a series of pro- 

 cesses are here commingled (see Fig. 99). The first leaves are small 

 and lanceolate ; and at the base of each a membranous tooth, or it may 

 be two such teeth, are distinguishable which are to be regarded as the 

 arrested stipules. As the leaf becomes larger its apex grows out into 

 a tendril which at first is rudimentary. Right and left of it, or perhaps 

 only on one side, a further differentiation takes place and two lateral tendrils 

 appear. These are seated then upon a broad leaf-surface. Subsequently 

 pinnules appear and the uppermost leaves have a bifid or trifid terminal 

 tendril with one or two pairs of pinnules below it ; not infrequently 

 a tendril may be observed opposite one of the pinnules. How are 

 we to explain this peculiar construction l ? Irmisch considered the flat- 

 tened portion of the leaf to be a leaf-stalk. This is however opposed to 

 the history of development and to what we find in other Leguminosae. 



P 

 I 



Z 



jr. 



FIG. 99. Lathyrus Clymenum. Forms of leaf from a seedling at different ages. / unrnembered primary leaf; 

 // primary leaf ending in a tendril, st rudimentary stipule ; /// and IV leaves upon which pinnules have appeared. 



In them we commonly find primary leaves of the form represented in 

 Fig. 94, /, and these are certainly arrested formations, as I have 

 experimentally proved a is not a leaf-stalk without a blade but 

 corresponds to the whole upper portion of the primordium of a leaf, the 

 so-called upper leaf; a leaf-stalk is generally not yet developed. If we 

 imagine the portion marked a to be greatly increased and the portion 

 immediately below it to be only slightly developed, then we should arrive 

 at a condition as we find it in Lathyrus Ochrus (Fig. 99, /). In this 

 species however the leaf-formation remains long at this stage ; the surface 

 of the leaf is then not a leaf-stalk 2 but the whole of the upper part of 

 the primordium of a leaf no longer sharply separable from the lower 



1 An account of the development of leaves ought properly to precede the discussion of this matter, 

 but that falls into the special part of this book and I cannot for other reasons omit this peculiar 

 case here. 



2 Schenck, Beitrage zur Biologic und Anatomic der Lianen, i. p. 184, brings forward again the 

 explanation given by Irmisch, but- overlooks all developmental facts. 



