The Science of Plant Growing 



69 



and crustaceous in the Honeysuckle ; while it may be much wrinkled, 

 crustaceous, tubercled, or warted in species of Delphinium. Tubercled seeds 

 may be found in Lychnis, Silene, and Stellaria; netted or pitted ones in 

 Poppy, Passionflower, and their allies. In the Willow Herb (Epilobium) 

 the seeds develop a long pencil of hairs at the top, and in the Willow and 

 Poplar the parachute of hairs arises at the base; and in all cases the hairs 

 are intended to assist the dissemination of the seeds by the wind. This, 

 also, is the intention where the testa develops round the edges or at both 

 ends into a membranous expansion or wing, as in the Bignonia and Moon- 

 seed families, in the Stock, Arabis, and many others. 



As the seed matures, the endosperm 

 grows and becomes fleshy, horny, or 

 mealy in different families, and fills up 

 the interior of the seed to a greater or 

 less extent, or may disappear altogether, 

 as in the Crucifers and Rose families, 

 where the embryo uses it all up before 

 maturity. The embryo may remain 

 small or grow to fill the seed, and inter- 

 mediate stages are very numerous. It 

 may store all the food materials in itself 

 and the cotyledons may remain thin or 

 become fleshy. At maturity the seed 

 assumes various colours, such as red, 

 brown, white, or black, and these colours 

 are a sign that growth has ceased. The 

 testa dries up, but the endosperm and 

 embryo remain alive and retain theii* 

 protoplasm and other contents, though 

 the embryo alone is capable of resuming 

 growth in most cases. The Castor-oil 

 seed is an exception, for the endosperm 

 also grows during germination. 



Germination. The essentials to ger- 

 mination are air, moisture, and a suitable temperature. The oxygen of 

 the air is necessary for the purpose of respiration, to keep the embryo 

 alive even while it is resting in the dry seed; but before germination can 

 take place it must respire more freely to induce the chemical changes 

 necessary for the resumption of growth. Moisture is required to soften 

 the testa and other hard parts with which it may be surrounded, as well 

 as to swell up the tissues of the embryo and endosperm, where such is 

 present. A certain degree of heat is necessary before any growth can 

 take place, and this has to be determined in each particular case. The 

 seed of the Sycamore may germinate at freezing-point, but many other- 

 plants require a considerably higher temperature. On the other hand, 

 very few species of plants will germinate in a higher temperature than 



Fig. 60. Seeds with the Testa or Skin winged, 

 or provided with a tuft of Hairs, the object in 

 each case being to secure the dispersal of the Seed 



1, Lepigonum. 2, 3, Aspen. 4, 5, Cinchona. 



