SECTION U.] FRUIT. 117 



both kinds are fraitful. Houstonia and Mitchella, or Partridge-berry, are 

 excellent and familiar examples. These are cases of 



Heterogone Dimorphism, the relative lengths being only short and long 

 reciprocally. 



Heterogone Trimorphism, in which there is a mid-length as well as a long 

 and a short set of stamens and style; occurs in Lythrum Salioaria and some 

 species of Oxalis. 



342. There must be some essential advantage in cross fertilization or 

 cross breeding. Otherwise all these various, elaborate, and exquisitely 

 adjusted adaptations would be aimless. Doubtless the advantage is the 

 same as that which is realized in all the higher animals by the distinction 

 of sexes. 



§ 2. ACTION OF POLLEN, AND FORMATION OF THE EMBRYO. 



343. Pollen-growth. A grain of pollen may be justly likened to one 

 of the simple bodies (spores) which answer for seeds in Cryptogamous plants. 

 Like one of these, it is capable of germination. When deposited upon the 

 moist surface of the stigma (or in some cases even when at a certain dis- 

 tance) it grows from some point, its living inner coat breaking through the 

 inert outer coat, and protruding in the form of a delicate tube. This as it 

 lengthens penetrates the loose tissue of the stigma and of a loose conduct- 

 ing tissue in the style, feeds upon the nourishing liquid matter there pro- 

 vided, reaches the cavity of the ovary, enters the orifice of an ovule, and 

 attaches its extremity to a sac, or the lining of a definite cavity, in the 

 ovule, called the Finibryo-Sac. 



344. Origination of the Embryo. A globule Df living matter in the 

 embryo-sac is formed, and is in some way placed in close proximity to the 

 apex of the pollen tube ; it probably absorbs the contents of the latter ; it 

 then sets up a special growth, and the Embryo (8-10) or rudimentary 

 plantlet in the seed is the result. 



Section XIV. THE FRUIT. 



345. Its Nature. The ovary matures into the Pruit. Tn the strictest 

 sense the fruit is the seed-vessel, technically named the Pericarp. But 

 practically it may include other parts organically connected^ with tlie peri- 

 carp. Especially the calyx, or a part of it, is often incorporated with the 

 ovary, so as to be undistinguishably a portion of the pericarp, and it even 

 forms along with the receptacle the whole bulk of such edible fruits as 

 apples and pears. The receptacle is an obvious part in blackberries, and 

 is the whole edible portion in the strawberry. 



346. Also a cluster of distinct carpels may, in ripening, be consolidated 

 or compacted, so as practically to be taken for one fruit. Such are raspber- 



