COTYLEDONS. 601 



germination commences the two cotyledons grow iu length, penetrate through 

 this loose central cell-layer and attach themselves to the reserve-tissue which 

 has to be absoi'bed. 



On the whole it may be taken as correct that the surface of contact between 

 the absorbent part and that which has to be absorbed is greater the quicker 

 the absorption has to be accomplished, on account of the local climatic conditions. 

 Starch is best suited for rapid liquefaction and absorption ; fat takes much 

 longer to become changed into a form adapted for absorption; and the trans- 

 formation of layers of cellulose requires a still longer time. In accordance with 

 this the embryo comes into contact with the reserve -tissue whose cells are 

 crowded with starch (as, for example, in the seeds of pinks, oraches, poly- 

 gonums, and grasses), presenting a broad surface, or else wrapped with its long 

 cotyledons around the tissuf either horse-shoe-wise or spirally. On the other 

 hand, iu plants whose special reserve-tissue is principally filled with fat, the 

 surface in contact is much smaller, and the seeds of those plants whose reserve- 

 food consists chiefly of cellulose, e.g. those of the Date, usually exhibit only 

 a very limited area of contact between the cotyledon and the reserve-tissue. 

 But in these latter the liquefaction and absorption continue for mouths, while 

 the same processes in the starchy seeds of grasses and oraches are completed 

 in a few days. 



In addition to this first task of the cotyledons, which we have just 

 described, in many instances we liave a second function, viz. the extrusion of 

 the hypocotyl and its crowning bud from the interior of the seed-coats. After 

 the formation of the embryo from the parent plant, it remains quiescent for 

 a time, ami during tliis period appears to be protected in the most diverse 

 ways by coverings against the external dangers that might threaten its exist- 

 ence. When a special reserve-tissue is present, the embryo is frequently found 

 embedded in the centre of it, or hidden in its folds. The reserve -tissue is 

 often horn-like, or as hard as bone, as, for example, in the seeds of Date and 

 Coffee, and therefore an excellent protection is provided by this tissue for the 

 dormant embryo. In any case the embryo is suiTOunded by the seed-coat, which 

 may consist of a single or a double layer. In very many plants the seed is 

 also walled in by a non-dehiscent pericarp (or fruit - covering) and occasionally 

 by other structures. The seed-coat forms an envelope which allows of the 

 entrance of moisture into the interior only by a very restricted opening. It is 

 not flexible or extensible to a great degree, and consequently if the contents 

 swell up and the growth of the embryo begins, then the portion of the embryo 

 designed for further development must either find an exit through the above- 

 mentioned aperture or else it must burst through the husk; or both kinds of 

 escape may occur together. 



This process, in which the cotyledons take a very prominent jjart, is carried 

 on in a manner defined for everj^ species, but in different species by an incalculable 

 variety of methods. Occasionally larger alliances of the vegetable kingdom 



