90 



CASSELL'S POPULAR GAEDENING. 



quite sufficient to show whether the plant is a 

 Dicotyledon or a Monocotyledon, without seeing the 

 seedling at all. On the other hand, a botanist can 

 safely predicate from the presence of two or only 

 one seed-leaf, as the case may be, what the nature of 

 the leaf and flower is likely to be. Of course, allow- 

 ance must always be made for exceptions, but in this 

 case the proportion of exceptions to the rule is vei-y 

 small indeed. 



The cotyledons vary very considerably in appear- 

 ance in different plants, so much so in fact that it is 

 impossible to allude to aU their variations of form. 

 Those that are purely hereditary we may in this place 

 pass over, but those which, by their special adapta- 

 tion, facilitate the present life-work of the individual 

 plant need some mention. From this point of view 

 the most important thing to note is the leafy or the 

 fleshy character of the seed-leaf. In some cases, as 

 in Mustard, or in the Melon, and many others, the 

 seed-leaves are little different save in form from 

 ordinary leaves. (See Fig. 16.) In other cases, as 

 in the Acorn or Almond, they are thick, white, and 

 fleshy, so unlike ordinary leaves that by any casual 

 observer they would probably not be considered to 

 be leaves ; nevertheless their position, mode of growth, 

 and essential structure are those of leaves, so that 

 no doubt whatever exists as to their real nature. 

 This difference in character points to an important 

 diversity in office. 



The thick seed-leaves of the Acom or Bean are 

 filled with niltritive matter for the use of the young 

 seedling. How that insoluble matter is rendered 

 soluble and available as food has been indicated in 

 a previous chapter (page 24). The thin green seed- 

 leaves have no such store available, but they are so 

 constructed as to be able to acquire food for them- 

 selves directly they are placed under conditions to 

 obtain it. These differences are thus obvipusly con- 

 nected with the circumstances, that as a rule the thick, 

 fleshy, white cotyledons, as of Acorns, &o., remain 

 below ground (hypogeal), while the thin leafy ones 

 are invariably pushed up into the air and light, to 

 put their green colouring matter into use at once in 

 a manner we shall have to speak of in considering 

 the office of the leaves (Pig. 16). The seed-leaves of 

 the French Bean are thick, but they are often raised 

 above the surface, and thus seem to be exceptional ; 

 but it will generally be found in such cases that the 

 exception is more apparent than real, inasmuch as 

 such seed-leaves, athough thick, are green or be- 

 come so. 



Movements of tlie Seed-leaves The seed- 

 leaves, like all other gi-owing parts, are the subjects 

 of movements such as have been described in the 

 radicle and caulicle, but regulated by their position 



and attachments, as well as by external conditions — 

 temperature, &c. In addition to these movements- 

 are certain others, called by Darwin " sleep move- 

 ments," and which have been observed by him in a 

 large number of plants of different orders. The 

 effect of these movements is to place the two seed- 

 leaves, which previously approximate to the horizontal 

 position, in a more or less vertical direction, and 

 nearly or quite in apposition. The object of these 

 movements seems to be to protect the upper surface 

 of the seed-leaf and the plumule from the effects of 

 chilhng by radiation. When leaves which -usually 

 close at night were prevented from so doing, the- 

 amount of dew upon the leaves forcibly kept apart- 

 was found to be great, wHle there was little or none 

 on the closed leaves. " The position of the leaves, 

 at night affects their temperature through radiation 

 to such a degree that when exposed to a clear sky 

 during a frost it is a question of Ufe and death." 

 If this be so with ordinary leaves, it is likely to be- 

 so to a greater extent with the seed-leaves. Attention 

 to little points of detail of this character, at present 

 almost wholly unobserved, will doubtless serve to- 

 explain many peculiarities now vaguely set down to- 

 differences of constitution or habit, and will suggest 

 to the thoughtful gardener the appropriate treatment 

 in doubtful cases. 



Practical Inferences from the foregoing 

 Statements. — More obvious are the indications 

 afforded by the different character of the seed, thick 

 or thin -rinded, large or small, -with thick or with 

 thin seed-leaves, remaining below or thrust up- 

 above ground. 



By an attentive consideration of the requirements- 

 for germination in general, of the physical and 

 chemical changes that take place during the opera- 

 tion, and of the peculiarities of form and organi- 

 sation of different seedling plants, the gardener may 

 obviously gain many hints as to the best method 

 of insuring the germination of the seeds entrusted, 

 to him. 



The matter has also an important bearing on the 

 question of adulteration or -wilful seed-killing ; thus, a 

 perfectly sound sample placed under suitable condi- 

 tions may be expected to produce 90 — 95 percent, of 

 seedlings, or, in fact, not one may fail. The same 

 seeds put into the hands of an unskilful practitioner, 

 or in less favourable conditions, may not grow more 

 than 50 per cent., and this from no fault of the seeds- 

 man, and no defect in the seed. 



The variation observable in the length of time re- 

 quired for germination depends also, doubtless, in a. 

 measure, upon the condition of ripeness of the seed.' 

 Thus, it has been shown that seeds of peas, beans, 

 wheat, &c., ripen before the seed-vessel containing 



