416 PART IV. CLASSIFICATION. 



Flower. The Willow has a funicular aril in the form of a tuft 

 of woolly hairs. The most striking example of a membranous 

 micropylar aril is the Spindle-tree (Euouymus) : in Euphorbia and 

 Polygala the micropylar aril is a small mass of tissue, and in 

 Asclepias it is a tuft of hairs. Other excrescences, not especially 

 connected with either the hilum or the micropyle (sometimes dis- 

 tinguished as caruncles or strophioles), occur in certain plants ; 

 thus in the Violet and the Celandine (Chelidonium) an elevated 

 ridge marks the course of the raphe, and in the Willow-herb 

 (Epilobium) a tuft of hairs springs from the chalaza. 



The most important point to be considered is, however, that of 

 the structural conditions which determine the production of a seed 

 in the Phanerogams, the feature which sharply defines this group 

 of plants from all others. The structural conditions are briefly as 

 follows : -the macrospore (embryo-sac) is not set free from the 

 macrosporangium (ovule), as is the case in the heterosporous 

 Pteridophyta ; nor does the macrosporangium itself separate from 

 the plant producing it until it has ripened into the seed : this 

 being so, the macrospore germinates inside the macrosporangium, 

 producing there the female prothallium with its reproductive 

 organs : fertilisation of the oosphere, as also the development of 

 the embryo from the oospore, takes place inside the macrospore; 

 and thus the seed is formed. If the macrospore were set free 

 from the macrosporangium, no seed would be formed ; but in 

 that case the condition of things would be that which actually 

 exists in the higher heterosporous Pteridophyta, such as Sela- 

 ginella and Isoetes. 



Some seeds can germinate as soon as they are shed : but, for 

 the most part, they only do so after a period of quiescence, though 

 they may lose their germinating power if this period be too pro- 

 longed. 



The Dissemination of the Seed. Fruits are either dehiscent, so 

 that the seeds escape, or they are indcliiscent : in the former case the 

 seeds, and in the latter case the fruits, present various adaptations 

 for ensuring their dispersal. The most conspicuous are those 

 which ensure dispersal by the wind : of this nature are the wing- 

 like appendages of the fruit in the Maple, Ash, Elm, etc. ; and 

 of the seed of Pinus, Catalpa, etc. : also the hairy appendages of 

 fruits (e.g. the pappus of Composite, the feathery style of Clema- 

 tis, etc.), and of seeds (e.g. on those of Grossypium the Cotton-plant, 

 Willow, Poplar, etc.). Other adaptations ensure dispersal by 



