31 



have observed its germination and development. The cucumber seed is 

 of medium size, and not particularly well supplied with an over-abun- 

 dance or great variety of reserve material for independent growth, as 

 shown by its habit of throwing up its cotyledons quite early in its period 

 of development. The cotyledons are withdrawn from the seed coat at 

 an early period, and as soon as they become exposed to the light they 

 turn green. At this period they commence to assimilate food from the 

 air, through the ability of the green cotyledons to decompose the at- 

 mospheric carbon-dioxide under the influence of light. 



The principal reserve materials which seeds contain are starch, pro- 

 teids and oils. All seeds, however, are not supplied with these three 

 reserve material constituents, but, on the other hand, Ihey may possess 

 only two of these, such as proteids and oils. Such seeds as the i>ea, 

 horse bean, etc., which contain an abundant supply of the three kinds of 

 reserve material, do not lift their cotyledons into the air, but remain sub- 

 merged ; whereas such plants as the sunflower, white lupine, etc , that 

 contain only proteids and oil (which constitutes a one-sided and incom- 

 plete supply of reserve material), immediately push their cotyledons 

 above ground The reserve material endowment of this latter class of 

 plants causes them to unfold their cotyledons and expose them to the 

 light, by which means they are enabled to assimilate food from the air) 

 thus supplying their inherent deficiencies. 



The seed of the cucumber belongs to this latter class, i.e., it contains 

 proteids and oil as a reserve material, but no starch Hence arises the 

 necessity of utilizing its cotyledons as assimilating organs at a very 

 early stage in its development, in order that the plant may be supplied 

 with carbohydrate material for metabolism and growth. Fig. 1 shows 

 an illustration of the seed of the cucumber and its various parts, to- 

 gether with the seedlings in difl'erent stages of development. Usually 

 one end of the seed is thinner and more pointed than the other. '1 he 

 pointed end is where the root protrudes during the process of gei-mina- 

 tion, and is that part of the seed where it is attached in the ovary or 

 fruit, as is shown by the scar, which is known as the hilum. When 

 seeds are soaked in water for a few hours, they swell up. If we squeeze 

 a water-soaked seed, it will be seen that water exudes at the hilum end. 

 This is because there is a pore at that end called the micropyle, which 

 enables the seed to absorb water readily. The seed is provided with 

 two coats : an inner one, which is thin and transparent ; and an outer 

 one, which is opacjue. Inside the coats is what is called the embryo, 

 which consists of two cotyledons, or seed leaves ; the radicle, or root ; 

 the hypocotyl, or that portion of the stem under the cotyledons ; and 

 the plumule, or developed stem. The embryo constitutes a minute 

 plantlet. Three ditt'erent stages of germination are also shown, and the 

 various parts which we saw in the embryo have become greatly extended 

 and developed. The cucumber plantlet has an ingenious and peculiar 

 way of getting out of its coats. The one-sided outgrowth (;j) near the 

 radicle and hypocotyl, known as a peg, acts as a lever in spreading the 

 micropyler end of the seed, by which means the cotyledons may be more 

 conveniently withdrawn. 



