° PHANEROGAMIA. 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. 
452, The larger reproductive cells are likewise produced 
