PMANEROGAMIA. 213 



447. The fibro-vascular bundles are of the collateral form, 

 the only exception being the first-formed bundle in the 

 root, which is of the radial type. The bundles are sym- 

 metrically arranged in the stem, through which they run 

 nearly parallel to each other, and extend into the leaves; 

 a few, however, have no connection with the leaves. 



448. All the kinds of tissues, with the exception of thick- 

 angled tissue, may occur in the bundles; but they are 

 mainly made up of tracheary, sieve, and fibrous tissues. In 

 the larger perennials, as the trees, the great mass of tissue 

 in the woody stems is principally made up of the tracheary 

 and fibrous tissues of the fibro-vascular bundles. In succu- 

 lent plants, especially those growing in water, the bundles 

 are usually smaller and more simple, being sometimes re- 

 duced to a thread of tracheary or sieve tissue. 



449. Of the remaining tissues, soft, tissue, in its various 

 forms, is by far the most common. The hypodermal por- 

 tions are frequently composed of thick-angled or stony 

 tissue. Milk-tissue is common in certain orders. 



450. The organs of reproduction in all flowering plants 

 are modifications of the type found in the higher Fern- 

 worts. The leafy plant produces two kinds of cells, an- 

 swering to the two kinds of spores we have lately studied. 

 Moreover, these reproductive cells are produced, as in 

 Fernworts, upon more or less modified leaves. 



451. The small reproductive cells, which are here called 

 pollen-cells instead of spores, develop in great numbers 

 within sac-like enlargements upon certain modified leaves. 

 They are set free by the breaking of the sac, and then 

 mostly fall out and are borne away by the winds, by in- 

 sects, or other means. 



462. The larger reproductive cells are likewise produced 



