Leaves and their modified forms

How these*forms and functions are varied to meet the necessities of plants in differ ent environments is now to be considered.All leaves may be divided into t v .7 o groups: seed leaves and shoot leaves.Seed leaves or cotyledons are those existing ready formed in the seed as a part of the embryo.Shoot leaves include the various forms found on stems,both sub terranean and aerial.That cotyledons are one form of leaf is evident from the fact that they often develop into true foliage leaves.This development is seen in the seedlings of the bean and squash.The once white^fleshy seed leaves,brought into the sunlight by the lengthening of the hypocotyl,become thin,green,expanded foliage leaves.Pig. 4 shows the squash seedling after several weeks growth,in which time the cotyledons have taken on the form of a foliage leaf.In the seedling of the Bean (Fig. 7) the cotyledons do not become so much like the foliage leaf as in tne squash.Here, after most of the nourishment in them has been absorbed, they shrivel,dry and finally drop off.In these two instances,the cotyledons are brought above the ground by the hypocotyl and become leaf like,however,whether they are brought above the ground depends on the amount of nourishment stored in them.For example,in the Pea the cotyledons are small,rounded and gorged with n o u r i s hm ent,so that tney do not appear above ground.From the ex am ination of the seedlings of these plants and comparing them with many others,I have drawn the following conclusion: When the cotyledons are very fleshy,short and thick,they do not appear above the ground; when not so fleshy,but thinner and more elongated,they are brought above the ground, slightly approach the form of the foliage leaf but after loosing most of the nourishment be come shriveled and drop off; and in cotyledons containing very little nourishment or those with ncme;they are raised above the ground and become true leaf structures.The form of the cotyle don varies greatly.As to general outline it may be unequal,unsymmetrical,lobed,divided,auricled, sessile,or petiolate; as to position of the radicle accumbent or incumbent; and as to margin en tire, crenate or emarginate.Though the study of these forms is very interesting,we are more con cerned with their transition to the form of a foliage leaf.There is a great contrast not only between cotyledons and the final leaves but even to those by which they are immediately followed.In order to determine how nearly the cotyledon approaches the form of the ultimate foliage leaf and to trace any intermediate stages of transition I have examined a number of seedlings and will give the results of my observations.PISUM SATIVUM Cotyledons two,oval,obtuse,plano-convex,fleshy,sessile,pale green,bursting the testa during germination,not expanded,strictLy subterranean,conforming to the outline of seed.Leaves of first two or three joints,very small scales followed by two or three simple leaves.Ultimate leaves compound,commonly two pairs of leaflets,ending in branching tendrils, smooth,glaucous,entire,elliptical, apex obtuse, p inn#tely netted veined.(Fig,3) CUCUPBITA MAXIMA Cotyledons two,opposite,ovate,entire,obtuse,hairy,short petiole,pinnately veined.Leaves simple,alternate, reniform,slightly or obtusely 5 lobed,hairy,palmately nerved,petiole short,dark green.(Pig.4)FAGOPHYRUM ESCULENTUM.Cotyledons two,very leaf like,broadly ovate,entire,light green,long petiole,raised 2 or 3 inches above the ground before extruded from seed coats.Leaves simple,alternate,triangular heart-shaped inclining to halberd-shaped or arrow shaped,long petioles,entire,smooth, light-green,stipules in the form of membraneous sheaths, CUCUMIS SATIVUS.Cotyledons broadly oblong, obtuse, narrowed at base into short, broad petioles,pale greer ascending nerves,entire,glabrous.Leaves simple,alternate,petiolate,hairy• No,l Lamina broadly cordate,palmately netted with 5 nerves,acute,irregularly den tate, slightly lobed,pale green above,paler beneath.Nos.Sand 3.More plainly^ 5 lobed.No. 4. Deeply 7 lobed and 7 nerved,lobes acuminate.(Pig.5)ACACIA DIALBATA Cotyledons sessile,oblong,entire,light green, obscurely nerved with prominent midrib, revolute,6 -7mm.long.Leaves, No. 1. Pinnate;leaflets linear oblong,sessile,pinnately nerved,entire,obtuse. No. 2 .Bipinnate;leaflets linear,obtuse entire,light green.(Fig. 6) PHASBOLUS VULGARIS Cotyledons fleshy,oblong, obtuse,entire, thick,longitudinally wrinkled,carried above ground by hypocotyl,never foliaceous,soon drop off.o -Leaves,leaflets First pair.opposite,unifolielate;triangular,reticulate,obtuse,5 nerved.