SCALE-LEAVES, FOLIAliE-LEAVES, FLOKAL-LEAVES. 623 



to bo expected that these green cotyleilous, whose function is precisely the same as 

 tliat of the green leaves of the shoot, should also be protected in exactly the same 

 way against external injurious intluences, and as a matter of fact many of the pi'otec- 

 tive contrivances are found on them which have been described in detail previously. 

 The cotyledons of many Boragineje are beset with stiff" bristles {e.g. Borago, 

 Caccinia, Anchusa, Myosotis (see tig. 148 *); those of roses are fringed with glandular 

 haii-s (see fig. 148'"); and those of many nettles bear stinging hairs on their upper 

 surface. It has been already pointed out on p. 350 that cot3dedons protect 

 themselves, and the young shoot-leaves hidden between them, against the injury 

 which might happen from loss of heat on clear nights by folding together and 

 assuming a vertical position. 



SCALE-LEAVES, FOLIAGE-LEAVES, FL0KAL-LEA\T:S. 



When the leaves borne on the shoot were distinguished as scale-leaves, foliage- 

 leaves, and floral-leaves, it was not implied that these three kinds of leaf -structures 

 were actually developed on all shoots. Scale-leaves are only found developed on 

 perennial plants. In annual plants they are entirely absent. Even the bud which 

 arises at the apex of the hypocotyl of an annual plant begins at once with green 

 foliage-leaves, nor are traces of scale-leaves to be seen in the buds which are subse- 

 quently developed on the epicotyl. Now, what can be the cause of this difference 

 between annual and perennial plants? Obviously annual plants require no scale- 

 leaves. It is of great importance for them that they should develop fruits and seeds 

 in the short period of a single summer; for this they must manufacture the building 

 materials necessary by the help of their green foliage-leaves. A portion of the 

 building materials is employed in the formation of the embryo in the seed; another 

 part in the production of well-stocked food-reserves associated with the embryo. 

 The seeds become detached and scattered, whilst the parent plant which has 

 produced them shrivels up and dies. It leaves no buds behind to persist through 

 the winter and sprout next year; consequently any pro\'ision for the maintenance 

 of such buds would be superfluous. It is different in perennial plants, as the buds 

 formed Vjy them must be provided with the necessary reserve-food, and protected 

 during the period of inactivity, throughout the winter sleep and summer rest, 

 against extremities of cold and heat, from freezing, burning, and drying up. They 

 must also be protected as well as possible against the attacks of animals, and these 

 tasks are assigned to the scale-leaves, which serve on the one hand as storehouses 

 for reserve food-materials, and on the other as protective envelopes covering the 

 still short axis with its rudiments of foliage or floral-leaves. Of course no green 

 leaf-blades, and, generally speaking, no green tissue is i-equired for the fulfilment of 

 the.se functions. The brown or colourless leaf-dieath is sufficient for the purpose; 

 which explains why the scale-leaves appear on all shoots as pale, husky or scale-like 

 structures without green blades. Even the first bud of the plant arising at the 

 apex of the hypocotyl is provided in most perennial plants with jwile scaly leaves. 



