Cactus

Cactus (harvested from Cacti) is a block that was introduced Alpha mode and remains in Beta. The block has one texture put on all vertical sides of the block, with another on top, and a third on the bottom (which looks like a simpler version of logs). Cacti are found naturally on Sand that is not in standing water. The block can be harvested easily by either mining the block underneath or directly by hand, provided you don't get too close. When any entity, including players and mobs, touches a cactus, half a heart of damage is taken. However, they will not shear sheep.

Cacti grows only on Sand to a maximum height of three blocks, though higher stacks can placed by the player. Bone meal doesn't work on cacti and won't speed their growth. Cacti can rarely be found on a beach, but are abundant in desert biomes.

Cacti can only be placed either on sand or on other cactus blocks, with no horizontally adjacent blocks. Cacti can not be placed on Sandstone. Cacti can be placed in water directly horizontally adjacent to other blocks, but that whole stack will break apart if any other block is placed on the cacti stack above that layer, including the cacti growing naturally.

Cacti will destroy any items that come into contact with them (since items have no health), including harvested cacti blocks. If a player (or mob) dies by cacti it is possible that many of their dropped items will be destroyed. Take careful note when making traps with cacti that if you are trying to farm resources, you may wish to build a different trap so that your loot is not destroyed.

Cacti will grow if placed in water, provided they break the surface. Cacti placed in a one block depth of water will grow normally. A two block cacti stack placed at a two block depth will grow to the three block height. Any higher stacks at greater depths will need to be placed entirely by the player. An interesting use of this is that since cactus blocks are transparent, high stacks of them that break the surface can be used to extend sunlight deep underwater. Contrary to popular belief, as of Beta 1.4_01, this does still work, as any cacti stacked on other cacti underwater will immediately break when placed if the bottom cactus block is NOT on sand. However, if the bottom cactus block IS on sand, it will still work. This is because when the cactus grows from the bottom one/you place a block on the bottom cactus, it updates the first one, and it doesn't matter if it's surrounded by water- it just matters if it's sitting on sand and doesn't have blocks next to it.

Cacti do not need light to grow and are non-flammable. As such, they grow well in the Nether, provided the player also brings in sand.

When the spot a cactus is placed in becomes unsuitable (e.g. a block is placed next to them), it collapses into a cactus block that can be collected and re-placed. Each mined block of cactus drops one cactus block.

Due to a biome bug (Only confirmed in Multiplayer so far), as of Alpha 1.2.1_01 in the desert biome a cactus can spawn floating in midair one to two blocks off the ground. However, the game will still think there is a cactus block under it, so the block directly under the floating cactus block is impassable, and can only be destroyed by destroying the visible sand block under it, or by placing a non-solid block underneath the 'ghost cactus'.

If a cactus is growing on a block of sand, making that block fall (without destroying it) will destroy the cactus.

Cacti have also been known to spawn on clay. Once removed they cannot be replaced.

As of Beta 1.2, cactus can be cooked in a furnace to make cactus green dye.

Cacti can now be placed adjacent to sugar canes as long as the proper accommodations are provided:a sand block next to a dirt block next to a water source.

History
Cactus was first seen in Alpha 1.0.6.

Before Alpha 1.0.11,
 * Cacti could be placed anywhere, and did not have the requirement that the space around it be empty. This allowed cacti to be placed to form a damaging floor: both players and mobs take half a heart of damage when they walk on a cactus block. This made it useful for traps, such as a hallway with a cactus block floor, which would quickly kill any mobs that walk on it.
 * Mining cacti hurt the player.
 * The texture was different.

Farming
It is possible to make a fully automated cactus farm which, when complete, requires no further effort from the player to harvest the resources.

Trivia

 * It is possible to spawn on or directly next to a cactus as they reside on Sand thus taking damage.
 * It is possible to manually place cacti on top of other cacti, creating one cactus as tall as the map.
 * It is possible for a cactus to be generated on top of another cactus, which can lead to naturally occurring cacti that exceed their height limits. An example of an abnormally tall cactus can be observed at the seed "Sharpie" and the coordinates (-173, 78, 58).
 * It is possible to create two doors from one when placed next to a cactus atop a sand block.
 * Cactus uses the same sound files as wool when breaking and placing.
 * Cacti vaguely resemble Creepers at a glance, particularly in low-light conditions.
 * When dropped onto the world, the Cactus item is slightly larger than other blocks.
 * When a minecart hits a cactus block, the minecart is destroyed and dropped.
 * Cactus can be placed next to saplings, however the cactus will break when the tree grows

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